RunCreateRequest declared stream_resumable as None-only, but langgraph_sdk
defaults it to False (not None) and its payload filter only drops None, so
every SDK request carried "stream_resumable": false and was rejected with 422.
False means "no resumable stream", which is what DeerFlow already serves.
This broke every IM channel run (runs.stream and runs.create both send the
field; only runs.wait omits it) and any external langgraph_sdk client. The web
frontend was unaffected because its compatibility wrapper does not send it.
An explicit true still returns 422. The new regression test drives a real SDK
client to capture its default payload and posts that to the HTTP boundary, so a
future non-None SDK default cannot regress this silently.
Fixes#4466