feat(ai): support Gemini thinking levels

[rekram1-node]

Summary

• explicitly map Google AI SDK thinkingConfig into native Gemini options
• support thinkingBudget, includeThoughts, and thinkingLevel in any combination
• add native Gemini request lowering and public typing for thinking levels
• stop forwarding unrelated Google settings through the generic provider-options mapper
• make resolver fixtures use provider-specific source options

Testing

• AI package typecheck
• Core package typecheck
• 35 Gemini and provider-package tests
• 28 AI SDK mapping and resolver tests