Remove the deprecated thread/rollback API (#44915)

## What changed

Remove `thread/rollback`, its request and response types, generated bindings, and the core `Op::ThreadRollback` operation. Requests now follow the generic unknown-method rejection path. Document `thread/revert` as the alternative for paginated threads.

Keep historical `ThreadRolledBack` markers and legacy error deserialization so existing rollouts remain compatible with replay and migration.

## Testing

Adapt retained-context and Guardian history tests to append legacy rollback markers and resume threads, preserving coverage of surviving instructions, answers, and review history.

GitOrigin-RevId: b3da1becdf86b1869275aacb0ffc2817cee5af2e

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

@@ -1456,7 +1456,7 @@
           "description": "Optional analytics source classification for this thread."
         },
         "turns": {
-          "description": "Only populated on `thread/resume`, `thread/rollback`, `thread/fork`, and `thread/read` (when `includeTurns` is true) responses. For all other responses and notifications returning a Thread, the turns field will be an empty list.",
+          "description": "Only populated on `thread/resume`, `thread/fork`, and `thread/read` (when `includeTurns` is true) responses. For all other responses and notifications returning a Thread, the turns field will be an empty list.",
           "items": {
             "$ref": "#/definitions/Turn"
           },