test: focus adapter coverage on four priority sites

[jackwener]

Summary

• split adapter tests into a dedicated Vitest project and keep default unit tests focused on core runtime coverage
• limit focused adapter coverage to zhihu, twitter, reddit, and bilibili
• add missing reddit and bilibili adapter tests
• update CI and contributor docs to use the new test entry points

Validation

• npm run typecheck
• npm test
• npm run test:adapter
• npm run build
• npm run docs:build