Add app-server support for reverting paginated threads (#38440)

## What changed

- Add the experimental `thread/revert` request, which replaces a loaded paginated thread's durable history with the prefix before `beforeTurnId` while preserving the thread ID.
- Interrupt any active turn, reload the replacement history without unloading the thread, preserve mutable thread settings, and emit `thread/reverted`.
- Return backward pagination cursors for the retained turns and items, reject stale rollout paths after a revert, and document that local file changes are unaffected.

## Testing

- Cover history replacement, pagination, stale paths, missing turns, subsequent turns, active-turn interruption, and preserved thread settings.

GitOrigin-RevId: 435a8163ad4f310cc47219b436f6bdde21f2b83a

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

@@ -4477,6 +4477,26 @@
           "title": "Thread/closedNotification",
           "type": "object"
         },
+        {
+          "properties": {
+            "method": {
+              "enum": [
+                "thread/reverted"
+              ],
+              "title": "Thread/revertedNotificationMethod",
+              "type": "string"
+            },
+            "params": {
+              "$ref": "#/definitions/v2/ThreadRevertedNotification"
+            }
+          },
+          "required": [
+            "method",
+            "params"
+          ],
+          "title": "Thread/revertedNotification",
+          "type": "object"
+        },
         {
           "properties": {
             "method": {
@@ -20968,6 +20988,19 @@
         "title": "ThreadResumeResponse",
         "type": "object"
       },
+      "ThreadRevertedNotification": {
+        "$schema": "http://json-schema.org/draft-07/schema#",
+        "properties": {
+          "threadId": {
+            "type": "string"
+          }
+        },
+        "required": [
+          "threadId"
+        ],
+        "title": "ThreadRevertedNotification",
+        "type": "object"
+      },
       "ThreadRollbackParams": {
         "$schema": "http://json-schema.org/draft-07/schema#",
         "description": "DEPRECATED: `thread/rollback` will be removed soon.",