Resume a previously paused or crashed agent run

Resumes a run from its last checkpoint. The response is an SSE stream whose first event is run-started, followed by the same domain events as the original streaming endpoint (e.g. plan, step, answer for AutoRag).

Concurrency: each call rotates the run’s session id. If a second resume request arrives while a first is in flight, the first wrapper exits silently on its next checkpoint write; only the latest resume continues streaming events to its caller.

Terminal states are surfaced via HTTP status codes:

• 410 Gone: run is Completed (the final answer is in the body) OR Expired (its resume cache is gone or no longer loadable). The body carries the original question and, for Completed runs, the rendered result.
• 422 Unprocessable Content: run is in a Failed state
• 404 Not Found: run does not exist or is not owned by the caller