fix(cron): reject invalid schedule parameters at creation time

[yorkhellen]

Summary

• Extend _validate_schedule_for_add to validate all schedule kinds upfront: reject invalid/empty cron expressions (croniter.is_valid), non-positive every_ms, missing at_ms, and tz used with non-
  cron schedules
• Catch ValueError in CronTool._add_job and return a user-facing error string instead of letting the exception propagate into the tool execution flow
• Previously, an invalid cron expression like "not a cron" would silently create a job with next_run_at_ms=None that would never execute — the user/agent would see "Created job" with no indication of failure

Test plan

• 7 new service-layer tests: invalid cron expr, empty cron expr, every_ms <= 0, negative every_ms, missing at_ms, tz with every, tz with at
• 2 new tool-layer tests: invalid cron expr returns Error:, empty cron expr returns Error:
• All 88 cron tests pass