test(music-render): assert caplog in webhook network-error test (T4 follow-up)

Code review found: test 5 accepted caplog fixture but never asserted on it
— silent regression risk if logger.exception is removed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

services/music-render/tests/test_nas_client.py

@@ -77,3 +77,4 @@ def test_webhook_swallows_network_error(caplog):
     )
     # raise 안 하면 통과
     webhook_update_task("task-5", "processing", 10)
+    assert "task-5" in caplog.text