test: add cron plugin coverage

[sureshchouksey8]

/claim #71

Purpose

Adds focused Vitest coverage for the cron plugin slice from #71.

Tasks

• Cover cron utility parsing and next execution calculation.
• Cover CronPlugin middleware table initialization and callback route registration.
• Cover no-task alarm behavior and earliest same-time task activation.
• Cover addEvent payload insertion/rescheduling and async callback waitUntil behavior.

Verify

• npx pnpm@9.15.4 vitest run plugins/cron/index.test.ts passes locally.
• npx pnpm@9.15.4 exec prettier --write plugins/cron/index.test.ts completed with no changes after formatting.
• git diff --check passes locally.

Note: full vitest run currently has unrelated existing RLS test failures in src/rls/index.test.ts; the new cron test passes in that run.