test: cover worker entrypoint

[kanjk0olb0y-a11y]

/claim #71

Summary

• Add focused Vitest coverage for the Worker fetch entrypoint in src/index.ts, which was previously at 0% in the coverage report.
• Cover CORS preflight short-circuiting, Durable Object region/session setup, internal/external/hyperdrive source selection, PostgreSQL/MySQL/SQLite provider configuration, admin/client/JWT auth, websocket token handling, pre-auth plugin responses, interface route bypass, and top-level error responses.
• Test-only change; no production behavior changed.

Validation

• pnpm exec vitest run src/index.test.ts - 14 tests passed
• pnpm exec vitest run src/index.test.ts --coverage.enabled true --coverage.include src/index.ts --coverage.reporter text - passed; src/index.ts reports 100% statements, 86.2% branches, 100% functions, 100% lines
• pnpm exec prettier --check src/index.test.ts - passed
• git diff --check -- src/index.test.ts - passed
• pnpm exec vitest run - new src/index.test.ts passes; current main still has 4 pre-existing src/rls/index.test.ts failures unrelated to this entrypoint-only change

No UI demo video is included because this is backend/unit-test-only coverage with no visible UI behavior.