inspect: require earlier agent deadline for timeout fallback

Classify inspector timeouts only when the agent context has a deadline that is strictly earlier than the parent deadline, or when the parent has no deadline. Equal deadlines can mean the agent context inherited the parent deadline, so do not report those parent timeouts as per-inspector failures. This applies before both the preserved DeadlineExceeded sentinel path and the lost-sentinel string fallback.

Update tests for equal-deadline parent timeouts and strictly-earlier inspector timeouts.