fix: reject unsupported response schemas on Vertex AI

[medisean]

Closes #2889

Vertex AI structured output does not support arbitrary additionalProperties response schemas, but the client previously allowed them and could return empty dictionaries at runtime. Reject these schemas before sending the request, with an error that identifies the supported mode.

Validation:

• python3 -m compileall -q google/genai
• git diff --check
• The targeted pytest could not run locally because the environment is missing google.auth.

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