fix(checkpointer): return InMemorySaver instead of None when not configured (#1016) (#1019)

* fix(checkpointer): return InMemorySaver instead of None when not configured (#1016)

* fix(checkpointer): also fix get_checkpointer() to return InMemorySaver

Make all three checkpointer functions consistent:
- make_checkpointer() (async) → InMemorySaver
- checkpointer_context() (sync) → InMemorySaver
- get_checkpointer() (sync singleton) → InMemorySaver

This ensures DeerFlowClient always has a valid checkpointer.

* fix: address CI failure and Copilot review feedback

- Fix import order in test_checkpointer_none_fix.py (I001 ruff error)
- Fix type annotation: _checkpointer should be Checkpointer | None
- Update docstring: change "None if not configured" to "InMemorySaver if not configured"
- Ensure app config is loaded before checking checkpointer config to prevent incorrect InMemorySaver fallback

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/src/agents/checkpointer/async_provider.py

@@ -10,7 +10,7 @@ Usage (e.g. FastAPI lifespan)::
     from src.agents.checkpointer.async_provider import make_checkpointer
 
     async with make_checkpointer() as checkpointer:
-        app.state.checkpointer = checkpointer  # None if not configured
+        app.state.checkpointer = checkpointer  # InMemorySaver if not configured
 
 For sync usage see :mod:`src.agents.checkpointer.provider`.
 """
@@ -87,20 +87,22 @@ async def _async_checkpointer(config) -> AsyncIterator[Checkpointer]:
 
 
 @contextlib.asynccontextmanager
-async def make_checkpointer() -> AsyncIterator[Checkpointer | None]:
+async def make_checkpointer() -> AsyncIterator[Checkpointer]:
     """Async context manager that yields a checkpointer for the caller's lifetime.
     Resources are opened on enter and closed on exit — no global state::
 
         async with make_checkpointer() as checkpointer:
             app.state.checkpointer = checkpointer
 
-    Yields ``None`` when no checkpointer is configured in *config.yaml*.
+    Yields an ``InMemorySaver`` when no checkpointer is configured in *config.yaml*.
     """
 
     config = get_app_config()
 
     if config.checkpointer is None:
-        yield None
+        from langgraph.checkpoint.memory import InMemorySaver
+
+        yield InMemorySaver()
         return
 
     async with _async_checkpointer(config.checkpointer) as saver: