fix(gateway): enforce thread ownership on stateless run endpoints (#3473)

POST /api/runs/stream and /api/runs/wait accept thread_id in the request
body but performed no owner authorization, letting any authenticated user
start runs on -- and read /wait checkpoint channel_values from -- another
user's thread (cross-user IDOR, #3472).

The @require_permission(owner_check=True) decorator resolves ownership from
the thread_id *path* param, so it cannot cover these body-param endpoints.
Enforce ownership inside start_run() before create_or_reject via
ThreadMetaStore.check_access: missing rows (auto-created temp threads) and
NULL-owner rows stay accessible, while a thread owned by another user
returns 404 (matching thread_runs.py). The internal system role (IM
channels acting for platform users) is exempt.

Closes #3472

backend/app/gateway/services.py

@@ -315,6 +315,21 @@ async def start_run(
                 detail=f"Model {model_name!r} is not in the configured model allowlist",
             )
 
+    # Stateless run endpoints carry thread_id in the request *body*, so the
+    # @require_permission(owner_check=True) decorator -- which resolves ownership
+    # from the path param -- cannot protect them. Enforce thread ownership here,
+    # before any run is created, so one user cannot start runs on (or read /wait
+    # checkpoint state from) another user's thread. Missing rows (auto-created
+    # temp threads) and NULL-owner rows (shared / pre-auth data) stay accessible
+    # via check_access; only a thread already owned by another user is rejected
+    # with 404, matching thread_runs.py's anti-enumeration behaviour. Internal
+    # channel runs act on behalf of IM users they do not own (see
+    # inject_authenticated_user_context), so the internal system role is exempt.
+    user = getattr(request.state, "user", None)
+    if user is not None and getattr(user, "system_role", None) != INTERNAL_SYSTEM_ROLE:
+        if not await run_ctx.thread_store.check_access(thread_id, str(user.id)):
+            raise HTTPException(status_code=404, detail=f"Thread {thread_id} not found")
+
     try:
         record = await run_mgr.create_or_reject(
             thread_id,