chore(app-server): add experimental annotation to relevant fields (#10928)

These fields had always been documented as experimental/unstable with
docstrings, but now let's actually use the `experimental` annotation to
be more explicit.

- thread/start.experimentalRawEvents
- thread/resume.history
- thread/resume.path
- thread/fork.path
- turn/start.collaborationMode
- account/login/start.chatgptAuthTokens

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

@@ -86,11 +86,6 @@
         "null"
       ]
     },
-    "experimentalRawEvents": {
-      "default": false,
-      "description": "If true, opt into emitting raw response items on the event stream.\n\nThis is for internal use only (e.g. Codex Cloud). (TODO): Figure out a better way to categorize internal / experimental events & protocols.",
-      "type": "boolean"
-    },
     "model": {
       "type": [
         "string",