Add `ResolveReferenceMetadata` to parse `include_metadata` /
`metadata_fields` from request and config payloads.
### Changes
- **New**: `internal/common/reference_metadata.go` — pure function, zero
dependencies
- **New**: `internal/common/reference_metadata_test.go` — 8 test cases
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>