fix: remove stale 'model' assertion from delegate_task schema test

The 'model' property was removed from DELEGATE_TASK_SCHEMA but the
test still asserted its presence, causing CI to fail.

tests/tools/test_delegate.py

@@ -56,7 +56,6 @@ class TestDelegateRequirements(unittest.TestCase):
         self.assertIn("tasks", props)
         self.assertIn("context", props)
         self.assertIn("toolsets", props)
-        self.assertIn("model", props)
         self.assertIn("max_iterations", props)
         self.assertEqual(props["tasks"]["maxItems"], 3)