test: cover cron plugin behavior

[newmattock]

Summary

• Adds focused Vitest coverage for CronPlugin registration, table initialization, no-task startup behavior, and callback route registration.
• Covers cron scheduling of the earliest matching tasks, active-task update parameters, and alarm setup.
• Covers addEvent persistence/rescheduling plus callback fanout and ExecutionContext.waitUntil handling for async event callbacks.

Refs #71
/claim #71

Tests

• pnpm exec vitest --run plugins/cron/index.test.ts -> 5 tests passed
• pnpm exec prettier --check plugins/cron/index.test.ts -> passed
• git diff --check -> passed
• pnpm exec vitest --run -> existing src/rls/index.test.ts baseline failures remain (4 RLS policy WHERE-clause expectations fail); the new cron test passes in that full run.