Ports the deleted v1 ProviderCommandReactor title coverage onto the v2
service: marker arming/clearing via thread.metadata.update, superseded
requestId no-ops, digest-driven regeneration, the "New thread" and
unchanged-title fallbacks, generation failure, and missing initial
messages, plus unit tests for formatThreadTitleContext.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>