codex: route metadata updates through ThreadStore (#20576)

- Route `thread/metadata/update` through
`ThreadStore::update_thread_metadata`.
- Add `LocalThreadStore` git metadata patch support for set, partial
update, and clear semantics.
- Add some unit tests for the new thread store code
- Remove a lot of dead code/tests!

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

@@ -11,6 +11,7 @@ use crate::CreateThreadParams;
 use crate::LoadThreadHistoryParams;
 use crate::LocalThreadStore;
 use crate::ResumeThreadParams;
+use crate::StoredThread;
 use crate::StoredThreadHistory;
 use crate::ThreadMetadataPatch;
 use crate::ThreadStore;
@@ -157,6 +158,20 @@ impl LiveThread {
         Ok(())
     }
 
+    pub async fn update_metadata(
+        &self,
+        patch: ThreadMetadataPatch,
+        include_archived: bool,
+    ) -> ThreadStoreResult<StoredThread> {
+        self.thread_store
+            .update_thread_metadata(UpdateThreadMetadataParams {
+                thread_id: self.thread_id,
+                patch,
+                include_archived,
+            })
+            .await
+    }
+
     /// Returns the live local rollout path for legacy local-only callers.
     ///
     /// Remote stores do not expose rollout files, so they return `Ok(None)`.