fix: correct isinstance check in webhook integration test

The `_get_finished_run_id` helper was checking `isinstance(run, ListOfRuns)`
after calling `.call()`, which returns a `Run` object, not `ListOfRuns`.
This caused flaky failures whenever no prior successful runs existed.

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

Author: vdusek

tests/integration/test_webhook.py

@@ -14,6 +14,7 @@
     ListOfRuns,
     ListOfWebhookDispatches,
     ListOfWebhooks,
+    Run,
     Webhook,
     WebhookDispatch,
     WebhookEventType,
@@ -39,7 +40,7 @@ async def _get_finished_run_id(client: ApifyClient | ApifyClientAsync) -> str:
     # No completed runs found - start one and wait for it to finish
     run = await maybe_await(client.actor(HELLO_WORLD_ACTOR).call())
 
-    assert isinstance(run, ListOfRuns)
+    assert isinstance(run, Run)
 
     return run.id