fix(python): make LangGraph sample entry points importable

[tryambak2019]

Summary

Restore the published HITL tools and kebab LangGraph sample entry points and validate their limited startup paths.

Implemented

• Use package-relative graph imports in both console CLIs.
• Extract build_app() functions so startup tests use the same ASGI construction path.
• Run both containers through their installed console entry points.
• Add CI coverage that mocks Uvicorn and verifies GET /health for each sample.

Validation

• OPENAI_API_KEY=fake make -C python test: 553 passed, 1 skipped.
• uv lock --check, Ruff lint, and Ruff format checks passed.
• HITL tools and kebab images build successfully.

#2689 Series

• Documentation corrections: docs(python): align integration READMEs with API v2 #2821
• Currency entry-point and startup validation: fix(python): make currency CLI entry point importable #2829
• Basic OpenAI agent entry-point and startup validation: fix(python): make basic OpenAI agent entry point importable #2830
• LangGraph HITL tools and kebab entry-point and startup validation: this PR
• Remaining ADK and AgentSTS audits, CrewAI validation, and residual cleanup: Revisit Python packages and samples after API v2 cutover #2689

Related to #2689. This PR does not close it.

[tryambak2019]

Superseded by #2887, which consolidates the requested Python startup and documentation changes for review.