fix(tests): convert create_mcp_client_session to asynccontextmanager

The multi-user-basic integration job was consistently failing with
`CancelledError: Cancelled via cancel scope ... by <async_generator_athrow>`
followed by a cascade of `anyio.ClosedResourceError` in every subsequent
test. Root cause: `create_mcp_client_session` was declared as an async
generator driven by `async for session in ...:`, so Python's generator
finalizer (`aclose`) ran under pytest-asyncio's cleanup task instead of
the task that owned the nested `streamablehttp_client` cancel scope.
anyio then raised when the inner task group saw its scope being exited
from a foreign task, leaving the memory object streams half-closed and
poisoning the rest of the session.

Switching to `@asynccontextmanager` + `async with ... as session:` makes
`__aenter__`/`__aexit__` run in the frame that owns the context manager,
satisfying anyio's structured concurrency requirements.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/integration/sampling_support.py

@@ -39,10 +39,10 @@ def create_sampling_callback(provider: Provider):
         if provider.supports_generation:
             callback = create_sampling_callback(provider)
 
-            async for session in create_mcp_client_session(
+            async with create_mcp_client_session(
                 url="http://localhost:8000/mcp",
                 sampling_callback=callback,
-            ):
+            ) as session:
                 # Session now supports sampling
                 pass
         ```