feat(app-server): add optional turn_id to thread/fork (#30277)

## Description

This adds stable optional `turnId` support to `thread/fork`. When
supplied, the fork copies persisted history through that terminal turn,
inclusive, and drops later turns from the new thread.

Omitting or passing `null` preserves the existing full-history fork
behavior, including the interruption marker when the stored source
history ends mid-turn.

## Why

We're deprecating `thread/rollback` and this will help certain UX use
cases work around it by using `thread/fork` + `turn_id` instead.

codex-rs/app-server-protocol/schema/json/ClientRequest.json

@@ -3673,6 +3673,13 @@
             }
           ],
           "description": "Optional client-supplied analytics source classification for this forked thread."
+        },
+        "lastTurnId": {
+          "description": "Optional last turn id to fork through, inclusive.\n\nWhen specified, turns after `last_turn_id` are omitted from the fork. The referenced turn cannot be in progress.",
+          "type": [
+            "string",
+            "null"
+          ]
         }
       },
       "required": [
@@ -6866,4 +6873,4 @@
     }
   ],
   "title": "ClientRequest"
-}
+}