test: improve coverage for resend and clerk plugins

[oocheol]

This PR contributes to bounty #71 by adding focused test coverage for previously uncovered core files and plugins:

• src/index.ts (Worker Entrypoint): Validates Cloudflare Worker fetch handler, CORS preflight, Durable Object initialization, external database configuration, and authentication flows.
• src/allowlist/index.ts: Validates query allowlist checks, admin bypass, and rejected query logging.
• plugins/sql-macros/index.ts: Validates 'SELECT *' prevention, admin bypass, and '' column expansion logic.
• Resend Plugin: Validates email sending logic, Authorization headers, and error handling.
• Clerk Plugin: Validates session existence check in the database and configuration validation.

/claim #71