fix: add __call__ to MockOptional for WTForms validator compatibility (apache#63823)

Closes: apache#63803

Author: YoannAbriel

airflow-core/src/airflow/api_fastapi/core_api/services/ui/connections.py

@@ -48,6 +48,9 @@ def __init__(
         ):
             pass
 
+        def __call__(self, form, field):
+            pass
+
     class MockEnum:
         """Mock for wtforms.validators.Optional."""
 

airflow-core/tests/unit/api_fastapi/core_api/services/ui/test_connections.py

@@ -0,0 +1,32 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+from __future__ import annotations
+
+from airflow.api_fastapi.core_api.services.ui.connections import HookMetaService
+
+
+class TestMockOptional:
+    def test_mock_optional_is_callable(self):
+        """MockOptional instances must be callable to satisfy WTForms validator checks."""
+        validator = HookMetaService.MockOptional()
+        assert callable(validator)
+
+    def test_mock_optional_call_is_noop(self):
+        """Calling MockOptional should be a no-op (returns None)."""
+        validator = HookMetaService.MockOptional()
+        result = validator(None, None)
+        assert result is None