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/tests/test_checkpointer.py

@@ -71,8 +71,13 @@ class TestCheckpointerConfig:
 
 
 class TestGetCheckpointer:
-    def test_returns_none_when_not_configured(self):
-        assert get_checkpointer() is None
+    def test_returns_in_memory_saver_when_not_configured(self):
+        """get_checkpointer should return InMemorySaver when not configured."""
+        from langgraph.checkpoint.memory import InMemorySaver
+
+        cp = get_checkpointer()
+        assert cp is not None
+        assert isinstance(cp, InMemorySaver)
 
     def test_memory_returns_in_memory_saver(self):
         load_checkpointer_config_from_dict({"type": "memory"})