feat: add Unbound as a provider

[vigneshsubbiah16]

Closes #7594

Adds Unbound as a provider to opencode. Docs: https://docs.getunbound.ai/

Changes:

• Added Unbound to bundled providers with custom loader
• Fetches available models dynamically from /models endpoint
• Parses model capabilities, token limits, and pricing
• Added X-Unbound-Metadata header for analytics
• Added provider icon

Configuration:

export UNBOUND_API_KEY=your-api-key

Verification:

• Tested opencode models lists Unbound models
• Tested chat completions with multiple models (gpt-4o-mini, claude-3-5-haiku, claude-sonnet-4-5)
• Added 10 unit tests covering all functionality

Post Merge: Add opencode support to the Unbound docs.