Clean up blocking-route overlay: remove dead code, colocate detection logic
- Remove dead `generateMetadata` branch from `BlockingPageLoadErrorDescription`
(never reached; metadata errors route through `DynamicMetadataErrorDescription`)
- Remove unused `apiName` field (regex never matched, always null)
- Colocate `getDynamicMetadataErrorDetails` into `blocking-route-error-details.ts`
- Make `isRuntimeVariant` private (single source, no longer exported)
- Fix grammar: "This indicate" → "This indicates"
- Tighten `refinement` type to `'' | 'generateViewport'`
Made-with: Cursor