test: cover worker auth context propagation

[jasperan]

Summary

• Add focused src/index.test.ts coverage for the Worker entrypoint: CORS preflight short-circuiting, admin bearer auth, websocket query-token auth, source/cache metadata, and regional Durable Object lookup.
• Fix a JWT auth context propagation bug where a verified JWKS payload was assigned after DataSource.context was created, so downstream RLS/context consumers could miss claims like sub.
• Keep production behavior scoped to copying the authenticated context onto the existing data source before delegating to StarbaseDB.handle.

Validation

• corepack pnpm exec prettier --check src/index.ts src/index.test.ts
• corepack pnpm exec vitest run src/index.test.ts
• corepack pnpm exec vitest run src/index.test.ts --coverage.enabled --coverage.include=src/index.ts --coverage.reporter=text
• git diff --check

Notes

• I also ran the full Vitest suite; it still reports existing unrelated src/rls/index.test.ts assertion failures around multi-table/subquery RLS rewriting. This PR only touches src/index.ts and adds src/index.test.ts.
• Non-UI/backend test change, so the verification commands above are the demo evidence.

/claim #71