Model HTTP retry/backoff + real background-job cancellation

[adamcohenhillel]

OpenAiResponsesAgentAdapter calls the model with 15s connect / 45s read timeouts but no retry/backoff on 429 or 5xx. OpenPhoneAgentJobScheduler.isCancelled() hardcodes false (:142-144), so background jobs can't stop while waiting on a stuck upstream.

Fix

• Bounded exponential backoff with jitter for 429/5xx.
• Respect Retry-After when present.
• Wire isCancelled() to a real signal from the job manager so a user-cancelled job actually stops.
• Add a RuntimeException-safe path so a broker outage doesn't kill the whole agent thread.

Size: M