Follow-up to #1686. Locks in two nits flagged during review:
1. agent.Result.Status doc comment now lists "cancelled" alongside the
existing values, so the enum surface matches actual usage.
2. New TestExecuteAndDrain_ContextCancelled_ReportsCancelled exercises
the path added in #1686: when the parent context is cancelled before
the backend produces a Result, executeAndDrain must return
Status="cancelled" (not "timeout"). A regression here would silently
restore the misleading log line we just fixed.