* fix(gateway): answer malformed run bodies with 4xx instead of 500
RunCreateRequest types config, context and input as free-form dicts, so
some bodies it accepts still failed during run admission with an
exception nothing mapped, and the run routes answered 500:
- a non-object config.configurable (build_run_config's dict.update, and
.get in the stateless thread selector)
- a non-object config.context (build_run_config's own ValueError)
- an assistant_id that does not normalize to letters, digits and hyphens
- a non-string agent_name in config.configurable, config.context or
context (_load_scope_agent_config)
- an AI message whose tool_calls is a string, object or list of strings
(AttributeError from LangChain's validator)
The request model now requires config.configurable and config.context to
be objects or null (422 invalid_run_config_section), and
build_run_config checks configurable as it already checked context.
start_run maps build_run_config's ValueError to 422, as it does for
validate_thread_id. Message conversion also maps AttributeError to its
existing 400, and a non-string agent_name gets the existing 422 for an
unresolvable agent. All of these fail before admission, so no run is
created.
* docs(changelog): reference #7362 for the malformed run body fix
* docs(changelog): note the configurable shapes #7362 now rejects
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
