app-server: ignore persist_extended_history param

[owenlin0]

Why

Taking a step to removing the persistExtendedHistory field. It's not scalable to be persisting so much data in the rollout file and returning it in the thread history.

When a client explicitly sends true, the server now tells that client the parameter is deprecated and ignored so the caller has a clear migration signal via the deprecationNotice notification.

What changed

• Keep the persist_extended_history / persistExtendedHistory field in the v2 protocol for compatibility, but document it as deprecated and ignored.
• Ignore the parameter in app-server thread/start, thread/resume, and thread/fork; those paths always use limited history persistence now.
• Stop treating persistExtendedHistory as a running-thread resume override mismatch.
• Emit a connection-scoped deprecationNotice when a request explicitly sets persist_extended_history: true.

Verification

• Added thread_start_deprecates_persist_extended_history_true to cover the deprecation notice.
• cargo test -p codex-app-server
• cargo test -p codex-app-server-protocol