feat: add controlled codex thread rollback

tests/fixtures/codex-app-server-runtime.mjs

@@ -569,6 +569,29 @@ rl.on("line", (line) => {
     return;
   }
 
+  if (message.method === "thread/rollback") {
+    send({
+      id: message.id,
+      result: {
+        thread: {
+          id: message.params?.threadId,
+          name: "rollback fixture thread",
+          turns: [
+            {
+              id: "rollback-private-turn",
+              status: { type: "completed" },
+              userInput: "private rollback turn text should not leak",
+            },
+          ],
+          rollbackApplied: {
+            numTurns: message.params?.numTurns,
+          },
+        },
+      },
+    });
+    return;
+  }
+
   if (message.method === "thread/read") {
     send({
       id: message.id,