* feat(gateway): support idempotent thread runs
Accept Idempotency-Key on thread-scoped create, stream, and wait endpoints, scoped by owner and thread before durable admission.\n\nRefs #5257.
* fix(gateway): handle idempotent run reuse on wait and stream
Reused store-only records have no local task. /wait now waits on the
bridge when it can observe the stream, and otherwise returns durable
status instead of a stale checkpoint. A reused terminal stream that has
been evicted emits gap/reload_durable_state. Replay is bound to the
original input and assistant_id.
* fix(gateway): 409 reused in-flight streams on this worker
A store-only running record on a process-local bridge has no owner
stream. POST /runs/stream used to subscribe anyway, which created an
empty log and waited forever. Match join: 409 unless the run is already
terminal, so missing-stream retries can still emit gap.
* fix(gateway): keep observer joins off the idempotent stream-gap path
sse_consumer keyed missing-stream gap on the sticky
idempotency_reused flag, so a later join of a terminal run inherited
it. Gate that branch on apply_on_disconnect, which already separates
creating streams from joins. Document the retry outcomes clients have
to handle.
* fix(gateway): gate missing-stream gap on creating retry
Reuse apply_on_disconnect to pick gap vs end changed sse_consumer default path, so a missing stream started returning gap for default callers and for out-of-scope POST /api/runs/stream. Keep that branch behind emit_gap_on_missing_stream and pass it only from thread-scoped /runs/stream on this request reuse.
* fix(gateway): keep wait reuse off later checkpoints
Direct handler calls were crashing because FastAPI Header() leaked in as the Python default. Bind Idempotency-Key with Annotated so the default is None, and ignore non-str keys.
A reused completed /wait was still reading the latest thread checkpoint. After a later run on the same thread that is the later run's result. Return durable status instead of claiming the head as this run's output.
* fix(gateway): snapshot wait reuse and refresh store status
idempotency_reused lives on the shared cached record. Capture it before awaiting completion so an overlapping retry cannot suppress the original creating /wait checkpoint.
A store-only peer record still holds admission-time status after the owner publishes END. Refresh durable status/error before returning them.
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