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/thread_metadata_sync.rs

@@ -382,7 +382,6 @@ mod tests {
     use pretty_assertions::assert_eq;
 
     use super::*;
-    use crate::ThreadEventPersistenceMode;
     use crate::ThreadPersistenceMetadata;
 
     #[test]
@@ -529,7 +528,6 @@ mod tests {
                 model_provider: "test-provider".to_string(),
                 memory_mode: ThreadMemoryMode::Enabled,
             },
-            event_persistence_mode: ThreadEventPersistenceMode::Limited,
         }
     }