app-server: remove experimental persist_extended_history bool flag (#25712)

## Summary

Remove the dead experimental `persistExtendedHistory` app-server flag
and collapse rollout persistence to the single policy app-server already
used.

## What Changed

- Removed `persistExtendedHistory` from v2 thread start/resume/fork
params and deleted its deprecation notice path.
- Removed the persistence-mode enums and plumbing through core, rollout,
and thread-store.
- Made rollout filtering mode-free, keeping the existing limited
persisted-history behavior.

## Test Plan

- `just write-app-server-schema`
- `cargo nextest run --no-fail-fast -p codex-app-server-protocol
schema_fixtures`
- `cargo nextest run --no-fail-fast -p codex-app-server
thread_shell_command_history_responses_exclude_persisted_command_executions`
- `cargo nextest run --no-fail-fast -p codex-rollout -p
codex-thread-store`
- final `rg` for removed flag/type names

codex-rs/thread-store/src/lib.rs

@@ -43,7 +43,6 @@ pub use types::StoredTurn;
 pub use types::StoredTurnError;
 pub use types::StoredTurnItemsView;
 pub use types::StoredTurnStatus;
-pub use types::ThreadEventPersistenceMode;
 pub use types::ThreadMetadataPatch;
 pub use types::ThreadPage;
 pub use types::ThreadPersistenceMetadata;