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/local/update_thread_metadata.rs

@@ -625,7 +625,6 @@ mod tests {
     use crate::ListThreadsParams;
     use crate::ResumeThreadParams;
     use crate::SortDirection;
-    use crate::ThreadEventPersistenceMode;
     use crate::ThreadMetadataPatch;
     use crate::ThreadPersistenceMetadata;
     use crate::ThreadSortKey;
@@ -788,7 +787,6 @@ mod tests {
                 history: None,
                 include_archived: true,
                 metadata: test_thread_metadata(),
-                event_persistence_mode: ThreadEventPersistenceMode::Limited,
             })
             .await
             .expect("resume external live thread");
@@ -1590,7 +1588,6 @@ mod tests {
                 history: None,
                 include_archived: true,
                 metadata: test_thread_metadata(),
-                event_persistence_mode: ThreadEventPersistenceMode::Limited,
             })
             .await
             .expect("resume archived live thread");