🧪 [testing improvement] Add error handling tests for calculateNextRun

🎯 What: The testing gap in the error handling of calculateNextRun (specifically when it fails to parse a cron expression) has been addressed. 📊 Coverage: The test suite for task-scheduler.test.ts now covers the happy paths (cron, interval, once), unknown schedule types, and the invalid cron expression error case. Additionally, tests for validateCronExpression were added. ✨ Result: Test coverage for src/task-scheduler.ts is significantly improved, catching real regressions if error logging or null-returns break during future refactors.

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PR created automatically by Jules for task 4419503267138658083 started by @LGZhss