fix(xai): treat response.incomplete as terminal success (#5113)

The xAI executor only switched on response.completed, so any turn that
ended with the spec-correct response.incomplete terminal event fell out
of the loop. Non-streaming requests were reported to the client as a 408
("stream disconnected before response.completed") even though the
upstream request succeeded, and because 408 is retryable it burned
credential rotations against a healthy pool. Streaming requests forwarded
the terminal event without patching the collected output items or
publishing usage.

Accept response.incomplete alongside response.completed in both paths,
mirroring the Codex executor, and keep the reasoning replay cache gated
on response.completed since a truncated turn has no replayable state.