Address semantic layer review nits

Improve semantic layer schema refresh error handling and connections endpoint behavior to reduce noisy failures while keeping this feature branch focused. Also restore frontend typing consistency and add debounce coverage for dynamic schema refresh.

tests/unit_tests/semantic_layers/api_test.py

@@ -1181,6 +1181,7 @@ def test_configuration_schema_enrichment_error_fallback(
 
     assert response.status_code == 200
     assert response.json["result"] == {"type": "object"}
+    assert response.json["warning"] == "connection failed"
     assert mock_cls.get_configuration_schema.call_count == 2
 
 
@@ -1218,9 +1219,11 @@ def test_connections_list(
 
     mock_db_session = mocker.patch("superset.semantic_layers.api.db.session")
     db_query = MagicMock()
+    db_query.options.return_value = db_query
     db_query.all.return_value = [mock_db]
     db_query.filter.return_value = db_query
     sl_query = MagicMock()
+    sl_query.options.return_value = sl_query
     sl_query.all.return_value = [mock_layer]
     sl_query.filter.return_value = sl_query
     mock_db_session.query.side_effect = [db_query, sl_query]
@@ -1252,28 +1255,7 @@ def test_connections_database_only(
     full_api_access: None,
     mocker: MockerFixture,
 ) -> None:
-    """Test GET /connections/ with semantic layers disabled."""
-    from datetime import datetime
-
-    mock_db = MagicMock()
-    mock_db.id = 1
-    mock_db.uuid = uuid_lib.uuid4()
-    mock_db.database_name = "PostgreSQL"
-    mock_db.backend = "postgresql"
-    mock_db.allow_run_async = False
-    mock_db.allow_dml = False
-    mock_db.allow_file_upload = False
-    mock_db.expose_in_sqllab = True
-    mock_db.changed_on = datetime(2026, 1, 1)
-    mock_db.changed_on_delta_humanized.return_value = "1 month ago"
-    mock_db.changed_by = MagicMock()
-    mock_db.changed_by.first_name = "John"
-    mock_db.changed_by.last_name = "Doe"
-
-    mock_db_session = mocker.patch("superset.semantic_layers.api.db.session")
-    db_query = MagicMock()
-    db_query.all.return_value = [mock_db]
-    mock_db_session.query.return_value = db_query
+    """Test GET /connections/ returns 404 when feature flag is disabled."""
 
     mocker.patch(
         "superset.semantic_layers.api.is_feature_enabled",
@@ -1282,12 +1264,7 @@ def test_connections_database_only(
 
     response = client.get("/api/v1/semantic_layer/connections/")
 
-    assert response.status_code == 200
-    assert response.json["count"] == 1
-    result = response.json["result"][0]
-    assert result["source_type"] == "database"
-    assert result["database_name"] == "PostgreSQL"
-    assert result["changed_by"]["first_name"] == "John"
+    assert response.status_code == 404
 
 
 @SEMANTIC_LAYERS_APP
@@ -1299,9 +1276,11 @@ def test_connections_name_filter(
     """Test GET /connections/ with name filter."""
     mock_db_session = mocker.patch("superset.semantic_layers.api.db.session")
     db_query = MagicMock()
+    db_query.options.return_value = db_query
     db_query.all.return_value = []
     db_query.filter.return_value = db_query
     sl_query = MagicMock()
+    sl_query.options.return_value = sl_query
     sl_query.all.return_value = []
     sl_query.filter.return_value = sl_query
     mock_db_session.query.side_effect = [db_query, sl_query]
@@ -1359,8 +1338,10 @@ def test_connections_sort_by_name(
 
     mock_db_session = mocker.patch("superset.semantic_layers.api.db.session")
     db_query = MagicMock()
+    db_query.options.return_value = db_query
     db_query.all.return_value = [mock_db]
     sl_query = MagicMock()
+    sl_query.options.return_value = sl_query
     sl_query.all.return_value = [mock_layer]
     mock_db_session.query.side_effect = [db_query, sl_query]
 
@@ -1412,6 +1393,7 @@ def test_connections_source_type_filter(
 
     mock_db_session = mocker.patch("superset.semantic_layers.api.db.session")
     db_query = MagicMock()
+    db_query.options.return_value = db_query
     db_query.all.return_value = [mock_db]
     mock_db_session.query.return_value = db_query
 
@@ -1454,6 +1436,7 @@ def test_connections_source_type_semantic_layer_only(
 
     mock_db_session = mocker.patch("superset.semantic_layers.api.db.session")
     sl_query = MagicMock()
+    sl_query.options.return_value = sl_query
     sl_query.all.return_value = [mock_layer]
     mock_db_session.query.return_value = sl_query