Add field to Thread object for the latest rename set for a given thread (#12301)

Exposes through the app server updated names set for a thread. This
enables other surfaces to use the core as the source of truth for thread
naming. `threadName` is gathered using the helper functions used to
interact with `session_index.jsonl`, and is hydrated in:
- `thread/list`
- `thread/read`
- `thread/resume`
- `thread/unarchive`
- `thread/rollback`

We don't do this for `thread/start` and `thread/fork`.

codex-rs/app-server-protocol/schema/json/v2/ThreadForkResponse.json

@@ -836,6 +836,13 @@
           "description": "Model provider used for this thread (for example, 'openai').",
           "type": "string"
         },
+        "name": {
+          "description": "Optional user-facing thread title.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "path": {
           "description": "[UNSTABLE] Path to the thread on disk.",
           "type": [