app-server: Move watch_id to request of fs/watch (#17026)

It's easier for clients to maintain watchers if they define the watch
id, so move it into the request.
It's not used yet, so should be a safe change.

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

@@ -3,7 +3,7 @@
   "description": "Stop filesystem watch notifications for a prior `fs/watch`.",
   "properties": {
     "watchId": {
-      "description": "Watch identifier returned by `fs/watch`.",
+      "description": "Watch identifier previously provided to `fs/watch`.",
       "type": "string"
     }
   },