feat: Add requestType tracking for header-negotiated markdown (#11585)
## Summary
- Add `requestType` parameter to `trackMdRequest`
- Defaults to `'md-url'` when not specified (backward compatible)
- Enables tracking of header-negotiated markdown requests when content
negotiation is implemented
## Related
- feedback-app PR: https://github.com/vercel/feedback-app/pull/176
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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>