Fix next-routing header interpolation (#91010)
## Summary
- interpolate source and `has` captures in `@next/routing` route headers
anywhere destinations are already interpolated
- make `onMatch` headers respect route matching and capture
interpolation instead of applying unconditionally
- keep middleware and initial request headers internal to routing so
`resolvedHeaders` only contains actual response headers
- add regressions for header interpolation and for request-header
leakage into `resolvedHeaders`
## Testing
- `pnpm --dir packages/next-routing types`
- `pnpm --dir packages/next-routing test -- --runInBand`
