fix(app-server): mark thread/turns/list and exclude_turns as experime… (#20499)

…ntal

We have some bugs to work out and it is not quite ready to consume as a
public API.

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

@@ -190,10 +190,6 @@
     "ephemeral": {
       "type": "boolean"
     },
-    "excludeTurns": {
-      "description": "When true, return only thread metadata and live fork state without populating `thread.turns`. This is useful when the client plans to call `thread/turns/list` immediately after forking.",
-      "type": "boolean"
-    },
     "model": {
       "description": "Configuration overrides for the forked thread, if any.",
       "type": [