fix(#252): migrate deprecated Gemini 3 preview endpoint

[mottych]

Summary

• Migrate deprecated Vertex endpoint usage from gemini-3-pro-preview to gemini-3.1-pro-preview in the model registry and Google Vertex provider allowlist.
• Upgrade google-genai dependency to current release (1.67.0) in both runtime and project dependency declarations.
• Refresh coaching/scripts/test_google_vertex.py to validate google-genai directly and default Vertex location to global for Gemini 3.1 model compatibility.

Incident Context

Issue #252 tracks Google Cloud's retirement of gemini-3-pro-preview, which would cause runtime failures after endpoint removal if left unchanged.

Online Research References

• https://docs.cloud.google.com/vertex-ai/generative-ai/docs/models/gemini/3-1-pro
• https://docs.cloud.google.com/vertex-ai/generative-ai/docs/models/gemini/3-flash
• https://pypi.org/project/google-genai/

Validation

• uvx ruff check coaching/src/core/llm_models.py coaching/src/infrastructure/llm/google_vertex_provider.py coaching/scripts/test_google_vertex.py
• uvx ruff format coaching/src/core/llm_models.py coaching/src/infrastructure/llm/google_vertex_provider.py coaching/scripts/test_google_vertex.py
• uv run mypy coaching/src/core/llm_models.py coaching/src/infrastructure/llm/google_vertex_provider.py --explicit-package-bases
• .venv\Scripts\python -m pytest coaching/tests/unit/infrastructure/llm/test_google_vertex_provider.py
• Preprod deployment + smoke tests: https://github.com/mottych/PurposePath_AI/actions/runs/23023408250
• Production post-deploy verification

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