fix(vllm_model): handle explicit null metadata in preprocessing

[rodboev]

responses_api_models/vllm_model can still crash on a valid request that explicitly sends "metadata": null, because preprocessing expects a dict and dereferences .get() on a None value. This PR keeps metadata=None on the same code path as omitted metadata by normalizing it to {} before any metadata lookups.

This PR fixes that with a narrow adapter-only change:

• Normalize only explicit metadata=None in _preprocess_chat_completion_create_params()
• Keep absent metadata absent and leave present non-null metadata payloads untouched
• Add a focused regression test that calls _preprocess_chat_completion_create_params() directly with metadata: None and asserts it returns with metadata == {}
• Keep the existing metadata override tests unchanged as non-regression coverage

No request schema changes, no other model adapters, and no dependency updates.

Closes #1096

Checklist

• Targeted tests pass: uv run pytest responses_api_models/vllm_model/tests/test_app.py -k "metadata_none or metadata_chat_template_kwargs_override or metadata_extra_body_override" -x
• Touched-file hooks pass: uv run pre-commit run --files responses_api_models/vllm_model/app.py responses_api_models/vllm_model/tests/test_app.py
• Touched-file lint passes: uv run ruff check responses_api_models/vllm_model/app.py responses_api_models/vllm_model/tests/test_app.py
• Touched-file format check passes: uv run ruff format --check responses_api_models/vllm_model/app.py responses_api_models/vllm_model/tests/test_app.py
• No request schema changes, other model adapter changes, or dependency changes
• DCO sign-off on all commits

The local validation commands above were attempted on this Windows host, but uv stops while resolving uvloop with RuntimeError: uvloop does not support Windows at the moment. Upstream CI is the source of truth for the full matrix.

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