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