fix(ci): follow-up test mock accepts user_id

Align run_store.start_run stub with the callback signature so the
offline refinement test passes in CI without BigQuery.

Author: samartho4

backend/eval/test_followup.py

@@ -41,7 +41,7 @@ def test_refine_creates_new_run() -> bool:
 
     started: list[str] = []
     orig_start = run_store.start_run
-    run_store.start_run = lambda goal, skill, run_id=None: (started.append(run_id), run_id)[1]  # no BQ
+    run_store.start_run = lambda goal, skill, run_id=None, user_id="": (started.append(run_id), run_id)[1]  # no BQ
     try:
         # (a) a FINISHED run is re-entered (a refinement was approved) -> a new run begins.
         state = {