test: cover cron plugin behavior

[Treasure520520]

Summary

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

This covers:

• plugin metadata and auth requirement
• next execution time calculation
• addEvent rejection before initialization
• middleware table initialization with no empty-task alarm scheduling
• nearest-task activation and setAlarm scheduling
• event insertion and alarm rescheduling
• /cron/callback payload dispatch
• async onEvent handlers being passed into ctx.waitUntil

Bounty

Contributes to Algora bounty issue #71.

Verification

• COREPACK_ENABLE_PROJECT_SPEC=0 corepack pnpm vitest plugins/cron/index.test.ts --run
• COREPACK_ENABLE_PROJECT_SPEC=0 corepack pnpm exec prettier --check plugins/cron/index.test.ts
• git diff --check
• COREPACK_ENABLE_PROJECT_SPEC=0 corepack pnpm exec tsc --noEmit --pretty false 2>&1 | tee /tmp/starbasedb-cron-tsc.log; ! rg -n "plugins/cron" /tmp/starbasedb-cron-tsc.log

Full-suite note:

• COREPACK_ENABLE_PROJECT_SPEC=0 corepack pnpm vitest --run still reports the existing 4 RLS failures in src/rls/index.test.ts; the new cron test file passes in that run.