Allow searching persisted attached threads (#60574)

## Why

Title and content searches exclude attached threads, preventing clients from finding their saved conversations alongside ordinary threads.

## What changed

- Add `includeAttachedThreads` to `thread/list` and `thread/search`. For `thread/list`, require a non-empty `searchTerm` and preserve the usual provider and source filters.
- Expose `attachedToThreadId` on thread responses to identify the owning parent independently of fork and subagent lineage.
- Support inclusive searches through SQLite and filesystem fallback, repairing missing database entries with saved thread names during title searches.

## Testing

Add regression coverage for opt-in title and content searches, attached-parent metadata after restart, default search filters, filesystem searches without SQLite, and discovery and repair of unindexed title matches.

GitOrigin-RevId: ac12ec3c730ad7f1722bdf8e21600d17f12017f0

codex-rs/cli/tests/queue.rs

@@ -474,6 +474,7 @@ async fn remote_session_commands_with_workload_identity_use_server_auth() -> Res
                 .request_typed(ClientRequest::ThreadList {
                     request_id: RequestId::Integer(4),
                     params: ThreadListParams {
+                        include_attached_threads: false,
                         current_account_only: false,
                         excluded_thread_ids: None,
                         attached_to_thread_id: None,