feat: add Google Gemini API parameters

[guillaumegay13]

Summary

• make MPS parameter paths authoritative provider API request paths with provider casing preserved
• add Google API-key parameter rows for gemini-2.5-pro, gemini-2.5-flash, gemini-2.5-flash-lite, and gemini-3.5-flash
• include generation length, sampling, thinking, and response MIME parameters using native generationConfig.* paths

Validation

• npm run validate
• npm test
• npm run typecheck
• npm run build
• npm run lint
• npm run guard:params
• npx prettier --check src/schema/model.ts tests/schema.test.ts docs/model-parameters-schema.md CONTRIBUTING.md models/google/*.yaml
• live Gemini generateContent smoke from YAML paths: gemini-2.5-flash-lite, gemini-2.5-flash, gemini-2.5-pro, gemini-3.5-flash all returned HTTP 200

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