[codex] Add local thread store listing (#17824)

Builds on top of #17659 

Move the filesystem + sqlite thread listing-related operations inside of
a local ThreadStore implementation and call ThreadStore from the places
that used to perform these filesystem/sqlite operations.

This is the first of a series of PRs that will implement the rest of the
local ThreadStore.

Testing:
- added unit tests for the thread store implementation
- adjusted some unit tests in the realtime + personality packages whose
callsites changed. Specifically I'm trying to hide ThreadMetadata inside
of the local implementation and make ThreadMetadata a sqlite
implementation detail concern rather than a public interface, preferring
the more generate StoredThread interface instead
- added a corner case test for the personality migration package that
wasn't covered by the existing test suite
- adjust the behavior of searched thread listing to run the existing
local rollout repair/backfill pass _before_ querying SQLite results, so
callers using ThreadStore::list_threads do not miss matches after a
partial metadata warm-up

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

@@ -5,12 +5,14 @@
 //! any other backing store.
 
 mod error;
+mod local;
 mod recorder;
 mod store;
 mod types;
 
 pub use error::ThreadStoreError;
 pub use error::ThreadStoreResult;
+pub use local::LocalThreadStore;
 pub use recorder::ThreadRecorder;
 pub use store::ThreadStore;
 pub use types::AppendThreadItemsParams;