[codex] Add rollout-backed thread content search (#23519)

## Summary
- add experimental `thread/search` for local rollout-backed thread
search using `rg` over JSONL rollouts
- return search-specific result rows with optional previews instead of
storing preview data on `StoredThread` or ordinary `Thread` responses
- keep `thread/list` separate from full-content search and document the
new app-server surface

## Testing
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-app-server
thread_search_returns_content_and_title_matches -- --nocapture`

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -17440,6 +17440,21 @@
         "title": "ThreadRollbackResponse",
         "type": "object"
       },
+      "ThreadSearchResult": {
+        "properties": {
+          "snippet": {
+            "type": "string"
+          },
+          "thread": {
+            "$ref": "#/definitions/v2/Thread"
+          }
+        },
+        "required": [
+          "snippet",
+          "thread"
+        ],
+        "type": "object"
+      },
       "ThreadSetNameParams": {
         "$schema": "http://json-schema.org/draft-07/schema#",
         "properties": {