Add Gemini 3 reasoning fetch example

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• Add fetch example for google/gemini-3-pro-preview reasoning
• Add comprehensive documentation at docs/gemini-thinking.md
• Update shared types to include reasoning and reasoning_details fields
• Example demonstrates:
  • Basic reasoning/thinking mode usage
  • Token budget configuration (max_tokens: 2000)
  • Accessing reasoning text and details
  • Token usage metrics

Model: google/gemini-3-pro-preview

• Max reasoning tokens: 200,000
• Context window: 1M tokens
• Reasoning: MANDATORY (always enabled)
• Created: 2025-11-18

Tested successfully:

• HTTP 200 response
• Reasoning tokens: 1,628
• Full reasoning process captured
• Structured reasoning_details present

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• #54
• #53 👈 (View in Graphite)
• main

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