feat: add Google Gemini subscription parameters

[guillaumegay13]

Summary

• add Google Gemini subscription MPS rows for the CodeAssist models used by Manifest's Gemini OAuth provider
• mirror the existing Gemini 2.5 API-key generationConfig params for the 2.5 subscription routes
• add Gemini 3 / 3.1 subscription rows with thinkingLevel-based reasoning controls where applicable

Sources

• Google Gemini GenerateContent generationConfig docs: https://ai.google.dev/api/generate-content
• Google Gemini thinking guide: https://ai.google.dev/gemini-api/docs/thinking
• Google Gemini 3 developer guide: https://ai.google.dev/gemini-api/docs/gemini-3
• Gemini 3.1 Flash-Lite model card: https://deepmind.google/models/model-cards/gemini-3-1-flash-lite/
• Model availability from Manifest PR feat(routing): add Gemini OAuth via CodeAssist manifest#1846 CodeAssist model refresh validation

Validation

• npm run validate
• npm test
• npm run build
• npm run typecheck
• npm run lint
• npm run guard:params

Live provider smoke: skipped in this repo because CodeAssist requires a Manifest-managed Google OAuth token/project binding. The model set is limited to the CodeAssist-recognized models already validated in mnfst/manifest#1846.

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