feat: add integration test case for flow execution

[weijinglin]

No description provided.

[coderzc]

Thanks for the contribution. I reviewed this PR and found two issues that should be addressed before merge:

1. [P1] New integration test is not executed in CI

• File: .github/workflows/hugegraph-llm.yml (Run integration tests step)
• test_flows_integration.py is added in this PR, but the workflow still runs only three explicit integration files. So the new tests are not part of PR checks and provide no regression protection.
• Suggestion: include the new file in the pytest command, or switch to directory-level collection with markers.

2. [P2] Missing external-service skip guard in new integration tests

• File: hugegraph-llm/src/tests/integration/test_flows_integration.py
• The tests call real flows directly (BUILD_VECTOR_INDEX / GRAPH_EXTRACT / RAG_* / TEXT2GREMLIN) without should_skip_external()-style gating or mocks. Once enabled in CI/local integration runs, they can become flaky and environment-dependent.
• Suggestion: add explicit skip conditions for external dependencies, or stabilize with mocks/stubs for non-deterministic external calls.

Please address these and I can re-review quickly.