test: cover resend plugin

[Treasure520520]

/claim #71

Summary

• add focused Vitest coverage for the Resend plugin
• cover constructor API-key storage, successful email request shape, authorization header, JSON body payload, provider error messages, fallback failure messages, and behavior when no API key is configured
• mock fetch so the tests do not require a real Resend account or network calls

Verification

• COREPACK_ENABLE_PROJECT_SPEC=0 corepack pnpm exec prettier --check plugins/resend/index.test.ts
• COREPACK_ENABLE_PROJECT_SPEC=0 corepack pnpm vitest plugins/resend/index.test.ts --run
• git diff --check
• COREPACK_ENABLE_PROJECT_SPEC=0 corepack pnpm exec tsc --noEmit --pretty false still reports existing repository errors in plugins/cdc, src/cache, src/do, and src/operation; no errors mention plugins/resend
• COREPACK_ENABLE_PROJECT_SPEC=0 corepack pnpm vitest --run still reports the existing 4 RLS failures in src/rls/index.test.ts; the new Resend test file passes in the full run

Notes

This is a non-overlapping coverage slice for #71, separate from the Stripe, WebSocket, Clerk, CDC, and SQL macros plugin PRs I submitted earlier.