Address @ShenAC-SAC's BUG-001 review on #3131. The previous cut still
stored an ``AppConfig`` snapshot on ``app.state.config`` for startup
bootstrap. Two follow-on hazards from that:
1. Future code touching the gateway lifespan could accidentally start
reading ``app.state.config`` again, silently regressing the request
hot path back to a stale snapshot.
2. ``get_run_context()`` paired a freshly-reloaded ``AppConfig`` with the
startup-bound ``event_store`` and a *live* ``run_events_config``
field — so an operator who edited ``run_events.backend`` mid-flight
would have produced a run context whose ``event_store`` and
``run_events_config`` referred to different backends.
Clean approach (aligned with the direction in PR #3128):
- ``lifespan()`` keeps a local ``startup_config`` variable and passes it
explicitly into ``langgraph_runtime(app, startup_config)`` and into
``start_channel_service``. No ``app.state.config`` attribute is set at
any point.
- ``langgraph_runtime`` now accepts ``startup_config`` as a required
parameter, removing the ``getattr(app.state, "config", None)`` lookup
and the "config not initialised" runtime error.
- The matching ``run_events_config`` is frozen onto ``app.state`` next
to ``run_event_store`` so ``get_run_context`` reads the two from the
same startup-time source. ``app_config`` continues to be resolved
live via ``get_app_config()``.
- ``backend/CLAUDE.md`` boundary explanation updated to spell out the
``startup_config`` / ``get_app_config()`` split.
New regression test ``test_run_context_app_config_reflects_yaml_edit``
exercises the worker-feeding path: it asserts that ``ctx.app_config``
follows a mid-flight ``config.yaml`` edit while
``ctx.run_events_config`` stays frozen to the startup snapshot the
event store was built from.
Refs: bytedance/deer-flow#3107 (BUG-001), bytedance/deer-flow#3131 review
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