Fix DeprecationWarning from httpx

Fix the following warning when running unit tests:

```
test/unit/webapps/test_request_scoped_sqlalchemy_sessions.py::test_request_scoped_sa_session_single_request
test/unit/webapps/test_request_scoped_sqlalchemy_sessions.py::test_request_scoped_sa_session_exception
test/unit/webapps/test_request_scoped_sqlalchemy_sessions.py::test_request_scoped_sa_session_concurrent_requests_sync
test/unit/webapps/test_request_scoped_sqlalchemy_sessions.py::test_request_scoped_sa_session_concurrent_requests_async
test/unit/webapps/test_request_scoped_sqlalchemy_sessions.py::test_request_scoped_sa_session_concurrent_requests_and_background_thread
  /usr/users/ga002/soranzon/software/nsoranzo_galaxy/.venv/lib/python3.10/site-packages/httpx/_client.py:1426: DeprecationWarning: The 'app' shortcut is now deprecated. Use the explicit style 'transport=ASGITransport(app=...)' instead.
```

See:

https://github.com/encode/httpx/blob/master/CHANGELOG.md#0270-21st-february-2024

test/unit/webapps/test_request_scoped_sqlalchemy_sessions.py

@@ -8,14 +8,18 @@
 import pytest
 from fastapi import FastAPI
 from fastapi.param_functions import Depends
-from httpx import AsyncClient
+from httpx import (
+    ASGITransport,
+    AsyncClient,
+)
 from starlette_context import context as request_context
 
 from galaxy.app_unittest_utils.galaxy_mock import MockApp
 from galaxy.webapps.base.api import add_request_id_middleware
 
 app = FastAPI()
 add_request_id_middleware(app)
+transport = ASGITransport(app=app)
 GX_APP = None
 
 
@@ -96,7 +100,7 @@ def assert_scoped_session_is_thread_local(gx_app):
 
 @pytest.mark.asyncio
 async def test_request_scoped_sa_session_single_request():
-    async with AsyncClient(app=app, base_url="http://test") as client:
+    async with AsyncClient(base_url="http://test", transport=transport) as client:
         response = await client.get("/")
         assert response.status_code == 200
         assert response.json() == {"msg": "Hello World"}
@@ -106,7 +110,7 @@ async def test_request_scoped_sa_session_single_request():
 
 @pytest.mark.asyncio
 async def test_request_scoped_sa_session_exception():
-    async with AsyncClient(app=app, base_url="http://test") as client:
+    async with AsyncClient(base_url="http://test", transport=transport) as client:
         with pytest.raises(UnexpectedException):
             await client.get("/internal_server_error")
         assert GX_APP
@@ -115,7 +119,7 @@ async def test_request_scoped_sa_session_exception():
 
 @pytest.mark.asyncio
 async def test_request_scoped_sa_session_concurrent_requests_sync():
-    async with AsyncClient(app=app, base_url="http://test") as client:
+    async with AsyncClient(base_url="http://test", transport=transport) as client:
         awaitables = (client.get("/sync_wait") for _ in range(10))
         result = await asyncio.gather(*awaitables)
         uuids = []
@@ -129,7 +133,7 @@ async def test_request_scoped_sa_session_concurrent_requests_sync():
 
 @pytest.mark.asyncio
 async def test_request_scoped_sa_session_concurrent_requests_async():
-    async with AsyncClient(app=app, base_url="http://test") as client:
+    async with AsyncClient(base_url="http://test", transport=transport) as client:
         awaitables = (client.get("/async_wait") for _ in range(10))
         result = await asyncio.gather(*awaitables)
         uuids = []
@@ -147,7 +151,7 @@ async def test_request_scoped_sa_session_concurrent_requests_and_background_thre
     target = functools.partial(assert_scoped_session_is_thread_local, GX_APP)
     with concurrent.futures.ThreadPoolExecutor() as pool:
         background_pool = loop.run_in_executor(pool, target)
-        async with AsyncClient(app=app, base_url="http://test") as client:
+        async with AsyncClient(base_url="http://test", transport=transport) as client:
             awaitables = (client.get("/async_wait") for _ in range(10))
             result = await asyncio.gather(*awaitables)
             uuids = []