chore: Update all generator integrations to use new generic callable (de)serializers for their callback handlers

[vblagoje]

We have migrated all the (de)serialize callback handlers in Haystack 2.x to use the new generic utils methods serialize_callable and deserialize_callable. Now, we need to do that for all generators in haystack-core-integrations. The best time to migrate these will likely be after the next Haystack beta is released.

[ZanSara]

Work to be done:

• Amazon Bedrock: chore: Update Amazon Bedrock integration to use new generic callable (de)serializers for their callback handlers #452
  • AmazonBedrockChatGenerator deserializes properly but fails to serialize the callback entirely 🤔
  • No need for AmazonBedrockGenerator, it uses a handler passed in the kwargs and doesn't serialize/deserialize it at all.
• Cohere: chore: Update Cohere integration to use new generic callable (de)serializers for their callback handlers #453
  • CohereGenerator needs work on to and from dict
  • CohereChatGenerator doesn't

All others don't need any change.

[masci]

@ZanSara I think the two linked PRs do something different - this issue was open to keep track of the changes needed after replacing haystack.components.generators.utils.serialize_callback_handler in main with haystack.utils.callable_serialization.serialize_callable. The course of action I imagine would involve a new beta release to ship serialize_callback_handler and change the integrations (or merging your current PRs but change the integration again later after we release the change). In that sense, you should add ollama to the list of integrations that we need to change.

[ZanSara]

For context, it's related to these PRs:

• feat: Externalize callable serialization so it can be reused haystack#6979
• fix: Reintroduce serialize_callback_handler and deserialize_callback_handler haystack#6988

Related PRs:

• Amazon Bedrock: chore: Use serialize_callable instead of serialize_callback_handler in Bedrock #459
• Cohere: chore: Use serialize_callable instead of serialize_callback_handler in Cohere  #460
• Ollama: chore: Use serialize_callable instead of serialize_callback_handler in Ollama #461

As mentioned, we need another beta release before merging these changes.