fix: resolve mypy type error in metrics push_to_gateway call

Narrow _REGISTRY type with an early return guard instead of assert
(which bandit flags as B101).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: JacobCoffee

cabotage/celery/metrics.py

@@ -101,6 +101,9 @@ def _push_metrics(stage: str, duration_seconds: float | None, labels: dict):
     if _STAGE_TOTAL is not None:
         _STAGE_TOTAL.labels(**labels, stage=stage).inc()
 
+    if _REGISTRY is None:
+        return
+
     try:
         push_to_gateway(
             pushgateway_url,