Implement stream() and astream() for agents (#12783)

```
---- chunk 1
{'actions': [AgentActionMessageLog(tool='Search', tool_input="Leo DiCaprio's current girlfriend", log="\nInvoking: `Search` with `Leo DiCaprio's current girlfriend`\n\n\n", message_log=[AIMessageChunk(content='', additional_kwargs={'function_call': {'name': 'Search', 'arguments': '{\n  "__arg1": "Leo DiCaprio\'s current girlfriend"\n}'}})])],
 'messages': [AIMessageChunk(content='', additional_kwargs={'function_call': {'name': 'Search', 'arguments': '{\n  "__arg1": "Leo DiCaprio\'s current girlfriend"\n}'}})]}
---- chunk 2
{'messages': [FunctionMessage(content="According to Us, the 48-year-old actor is now “exclusively” dating Italian model Vittoria Ceretti. A source told Us that DiCaprio is “completely smitten” with Ceretti, and their relationship is “going so well that Leo's actually being exclusive.”", name='Search')],
 'steps': [AgentStep(action=AgentActionMessageLog(tool='Search', tool_input="Leo DiCaprio's current girlfriend", log="\nInvoking: `Search` with `Leo DiCaprio's current girlfriend`\n\n\n", message_log=[AIMessageChunk(content='', additional_kwargs={'function_call': {'name': 'Search', 'arguments': '{\n  "__arg1": "Leo DiCaprio\'s current girlfriend"\n}'}})]), observation="According to Us, the 48-year-old actor is now “exclusively” dating Italian model Vittoria Ceretti. A source told Us that DiCaprio is “completely smitten” with Ceretti, and their relationship is “going so well that Leo's actually being exclusive.”")]}
---- chunk 3
{'actions': [AgentActionMessageLog(tool='Search', tool_input='Vittoria Ceretti age', log='\nInvoking: `Search` with `Vittoria Ceretti age`\n\n\n', message_log=[AIMessageChunk(content='', additional_kwargs={'function_call': {'name': 'Search', 'arguments': '{\n  "__arg1": "Vittoria Ceretti age"\n}'}})])],
 'messages': [AIMessageChunk(content='', additional_kwargs={'function_call': {'name': 'Search', 'arguments': '{\n  "__arg1": "Vittoria Ceretti age"\n}'}})]}
---- chunk 4
{'messages': [FunctionMessage(content='25 years', name='Search')],
 'steps': [AgentStep(action=AgentActionMessageLog(tool='Search', tool_input='Vittoria Ceretti age', log='\nInvoking: `Search` with `Vittoria Ceretti age`\n\n\n', message_log=[AIMessageChunk(content='', additional_kwargs={'function_call': {'name': 'Search', 'arguments': '{\n  "__arg1": "Vittoria Ceretti age"\n}'}})]), observation='25 years')]}
---- chunk 5
{'actions': [AgentActionMessageLog(tool='Calculator', tool_input='25^0.43', log='\nInvoking: `Calculator` with `25^0.43`\n\n\n', message_log=[AIMessageChunk(content='', additional_kwargs={'function_call': {'name': 'Calculator', 'arguments': '{\n  "__arg1": "25^0.43"\n}'}})])],
 'messages': [AIMessageChunk(content='', additional_kwargs={'function_call': {'name': 'Calculator', 'arguments': '{\n  "__arg1": "25^0.43"\n}'}})]}
---- chunk 6
{'messages': [FunctionMessage(content='Answer: 3.991298452658078', name='Calculator')],
 'steps': [AgentStep(action=AgentActionMessageLog(tool='Calculator', tool_input='25^0.43', log='\nInvoking: `Calculator` with `25^0.43`\n\n\n', message_log=[AIMessageChunk(content='', additional_kwargs={'function_call': {'name': 'Calculator', 'arguments': '{\n  "__arg1": "25^0.43"\n}'}})]), observation='Answer: 3.991298452658078')]}
---- chunk 7
{'messages': [AIMessage(content="Leonardo DiCaprio's current girlfriend is the Italian model Vittoria Ceretti, who is 25 years old. Her age raised to the 0.43 power is approximately 3.99.")],
 'output': "Leonardo DiCaprio's current girlfriend is the Italian model "
           'Vittoria Ceretti, who is 25 years old. Her age raised to the 0.43 '
           'power is approximately 3.99.'}
---- final
{'actions': [AgentActionMessageLog(tool='Search', tool_input="Leo DiCaprio's current girlfriend", log="\nInvoking: `Search` with `Leo DiCaprio's current girlfriend`\n\n\n", message_log=[AIMessageChunk(content='', additional_kwargs={'function_call': {'name': 'Search', 'arguments': '{\n  "__arg1": "Leo DiCaprio\'s current girlfriend"\n}'}})]),
             AgentActionMessageLog(tool='Search', tool_input='Vittoria Ceretti age', log='\nInvoking: `Search` with `Vittoria Ceretti age`\n\n\n', message_log=[AIMessageChunk(content='', additional_kwargs={'function_call': {'name': 'Search', 'arguments': '{\n  "__arg1": "Vittoria Ceretti age"\n}'}})]),
             AgentActionMessageLog(tool='Calculator', tool_input='25^0.43', log='\nInvoking: `Calculator` with `25^0.43`\n\n\n', message_log=[AIMessageChunk(content='', additional_kwargs={'function_call': {'name': 'Calculator', 'arguments': '{\n  "__arg1": "25^0.43"\n}'}})])],
 'messages': [AIMessageChunk(content='', additional_kwargs={'function_call': {'name': 'Search', 'arguments': '{\n  "__arg1": "Leo DiCaprio\'s current girlfriend"\n}'}}),
              FunctionMessage(content="According to Us, the 48-year-old actor is now “exclusively” dating Italian model Vittoria Ceretti. A source told Us that DiCaprio is “completely smitten” with Ceretti, and their relationship is “going so well that Leo's actually being exclusive.”", name='Search'),
              AIMessageChunk(content='', additional_kwargs={'function_call': {'name': 'Search', 'arguments': '{\n  "__arg1": "Vittoria Ceretti age"\n}'}}),
              FunctionMessage(content='25 years', name='Search'),
              AIMessageChunk(content='', additional_kwargs={'function_call': {'name': 'Calculator', 'arguments': '{\n  "__arg1": "25^0.43"\n}'}}),
              FunctionMessage(content='Answer: 3.991298452658078', name='Calculator'),
              AIMessage(content="Leonardo DiCaprio's current girlfriend is the Italian model Vittoria Ceretti, who is 25 years old. Her age raised to the 0.43 power is approximately 3.99.")],
 'output': "Leonardo DiCaprio's current girlfriend is the Italian model "
           'Vittoria Ceretti, who is 25 years old. Her age raised to the 0.43 '
           'power is approximately 3.99.',
 'steps': [AgentStep(action=AgentActionMessageLog(tool='Search', tool_input="Leo DiCaprio's current girlfriend", log="\nInvoking: `Search` with `Leo DiCaprio's current girlfriend`\n\n\n", message_log=[AIMessageChunk(content='', additional_kwargs={'function_call': {'name': 'Search', 'arguments': '{\n  "__arg1": "Leo DiCaprio\'s current girlfriend"\n}'}})]), observation="According to Us, the 48-year-old actor is now “exclusively” dating Italian model Vittoria Ceretti. A source told Us that DiCaprio is “completely smitten” with Ceretti, and their relationship is “going so well that Leo's actually being exclusive.”"),
           AgentStep(action=AgentActionMessageLog(tool='Search', tool_input='Vittoria Ceretti age', log='\nInvoking: `Search` with `Vittoria Ceretti age`\n\n\n', message_log=[AIMessageChunk(content='', additional_kwargs={'function_call': {'name': 'Search', 'arguments': '{\n  "__arg1": "Vittoria Ceretti age"\n}'}})]), observation='25 years'),
           AgentStep(action=AgentActionMessageLog(tool='Calculator', tool_input='25^0.43', log='\nInvoking: `Calculator` with `25^0.43`\n\n\n', message_log=[AIMessageChunk(content='', additional_kwargs={'function_call': {'name': 'Calculator', 'arguments': '{\n  "__arg1": "25^0.43"\n}'}})]), observation='Answer: 3.991298452658078')]}
```

libs/core/langchain_core/agents.py

@@ -1,9 +1,15 @@
 from __future__ import annotations
 
+import json
 from typing import Any, Literal, Sequence, Union
 
 from langchain_core.load.serializable import Serializable
-from langchain_core.messages import BaseMessage
+from langchain_core.messages import (
+    AIMessage,
+    BaseMessage,
+    FunctionMessage,
+    HumanMessage,
+)
 
 
 class AgentAction(Serializable):
@@ -34,6 +40,11 @@ def is_lc_serializable(cls) -> bool:
         """Return whether or not the class is serializable."""
         return True
 
+    @property
+    def messages(self) -> Sequence[BaseMessage]:
+        """Return the messages that correspond to this action."""
+        return _convert_agent_action_to_messages(self)
+
 
 class AgentActionMessageLog(AgentAction):
     message_log: Sequence[BaseMessage]
@@ -50,6 +61,20 @@ class AgentActionMessageLog(AgentAction):
     type: Literal["AgentActionMessageLog"] = "AgentActionMessageLog"  # type: ignore
 
 
+class AgentStep(Serializable):
+    """The result of running an AgentAction."""
+
+    action: AgentAction
+    """The AgentAction that was executed."""
+    observation: Any
+    """The result of the AgentAction."""
+
+    @property
+    def messages(self) -> Sequence[BaseMessage]:
+        """Return the messages that correspond to this observation."""
+        return _convert_agent_observation_to_messages(self.action, self.observation)
+
+
 class AgentFinish(Serializable):
     """The final return value of an ActionAgent."""
 
@@ -72,3 +97,69 @@ def __init__(self, return_values: dict, log: str, **kwargs: Any):
     def is_lc_serializable(cls) -> bool:
         """Return whether or not the class is serializable."""
         return True
+
+    @property
+    def messages(self) -> Sequence[BaseMessage]:
+        """Return the messages that correspond to this observation."""
+        return [AIMessage(content=self.log)]
+
+
+def _convert_agent_action_to_messages(
+    agent_action: AgentAction
+) -> Sequence[BaseMessage]:
+    """Convert an agent action to a message.
+
+    This code is used to reconstruct the original AI message from the agent action.
+
+    Args:
+        agent_action: Agent action to convert.
+
+    Returns:
+        AIMessage that corresponds to the original tool invocation.
+    """
+    if isinstance(agent_action, AgentActionMessageLog):
+        return agent_action.message_log
+    else:
+        return [AIMessage(content=agent_action.log)]
+
+
+def _convert_agent_observation_to_messages(
+    agent_action: AgentAction, observation: Any
+) -> Sequence[BaseMessage]:
+    """Convert an agent action to a message.
+
+    This code is used to reconstruct the original AI message from the agent action.
+
+    Args:
+        agent_action: Agent action to convert.
+
+    Returns:
+        AIMessage that corresponds to the original tool invocation.
+    """
+    if isinstance(agent_action, AgentActionMessageLog):
+        return [_create_function_message(agent_action, observation)]
+    else:
+        return [HumanMessage(content=observation)]
+
+
+def _create_function_message(
+    agent_action: AgentAction, observation: Any
+) -> FunctionMessage:
+    """Convert agent action and observation into a function message.
+    Args:
+        agent_action: the tool invocation request from the agent
+        observation: the result of the tool invocation
+    Returns:
+        FunctionMessage that corresponds to the original tool invocation
+    """
+    if not isinstance(observation, str):
+        try:
+            content = json.dumps(observation, ensure_ascii=False)
+        except Exception:
+            content = str(observation)
+    else:
+        content = observation
+    return FunctionMessage(
+        name=agent_action.tool,
+        content=content,
+    )