fix(cron): reject invalid one-shot timestamps

[lewismosciski]

Change Summary

ms-agent cron create '2025-02-30T09:00:00' 'test' currently saves a job that the scheduler silently skips forever. The parser checks only the timestamp prefix.

Validate one-shot timestamps with datetime.fromisoformat() before creating the schedule. Invalid dates, times and trailing text now raise ValueError; valid timestamps retain their original representation.

Validation: python -m pytest tests/cron -q (108 passed); changed-file pre-commit passed. CLI checks confirm that an invalid date exits with an error without saving a job, and a valid timestamp with a UTC offset is saved. Full-repo pre-commit has existing failures on main, including ms_agent/config/skills_manager.py:61 and ms_agent/permission/shell_validator.py:91.

Related issue number

None; found while checking the cron parser and scheduler.

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Run pre-commit install and pre-commit run --all-files before git commit, and passed lint check. Ran both; full-repo failures noted above.
• Documentation reflects the changes where applicable (no new command or configuration)