refactor(tests): remove redundant graph validation tests from WorkflowService unit tests

- Deleted tests for graph initialization and error propagation that were deemed unnecessary.
- Cleaned up the test suite to improve maintainability and focus on essential validation scenarios.

api/tests/unit_tests/services/workflow/test_workflow_service.py

@@ -1,8 +1,7 @@
-from unittest.mock import MagicMock, patch
+from unittest.mock import MagicMock
 
 import pytest
 
-from core.workflow.graph.validation import GraphValidationError, GraphValidationIssue
 from models.model import App
 from models.workflow import Workflow
 from services.workflow_service import WorkflowService
@@ -161,26 +160,4 @@ class TestWorkflowService:
 
         assert workflows == []
         assert has_more is False
-        mock_session.scalars.assert_called_once()
-
-    def test_validate_graph_structure_invokes_graph_init(self, workflow_service, mock_app):
-        graph = {"nodes": [], "edges": []}
-
-        with patch("services.workflow_service.Graph.init") as mock_graph_init:
-            workflow_service.validate_graph_structure(mock_app, graph)
-
-        mock_graph_init.assert_called_once()
-        assert mock_graph_init.call_args.kwargs["graph_config"] is graph
-        assert "node_factory" in mock_graph_init.call_args.kwargs
-
-    def test_validate_graph_structure_propagates_graph_errors(self, workflow_service, mock_app):
-        graph = {"nodes": [], "edges": []}
-        issue = GraphValidationIssue(code="ERR", message="invalid")
-
-        with patch("services.workflow_service.Graph.init", side_effect=GraphValidationError([issue])):
-            with pytest.raises(GraphValidationError):
-                workflow_service.validate_graph_structure(mock_app, graph)
-
-    def test_validate_graph_structure_requires_nodes_and_edges(self, workflow_service, mock_app):
-        with pytest.raises(ValueError, match="must include 'nodes' and 'edges'"):
-            workflow_service.validate_graph_structure(mock_app, {"nodes": []})
+        mock_session.scalars.assert_called_once()