Add WorkerFailureKind::Provider variant and observe_completion() method
to classify degraded session completions as structured failures.
- Detects finish='unknown' + zero tokens as provider failure
- Detects finish='error' as provider failure
- Normal completions transition to Finished state
- 2 new tests verify classification behavior
This closes the gap where sessions complete but produce no output,
and the failure mode wasn't machine-readable for recovery policy.
ROADMAP P2.13 backlog item added.