[CLEAN] Synthetic Benchmark PR #104004 - feat(crons): Always create detectors for all monitors in MonitorValidator

[ofir-frd]

Benchmark PR getsentry#104004

Type: Clean (correct implementation)

Original PR Title: feat(crons): Always create detectors for all monitors in MonitorValidator
Original PR Description: Previously, detectors were created as part of the endpoint, after detector → alerts linking happened. This changes it so detectors are crated immediately after the cron monitor is created.

Changes:

• Moved ensure_cron_detector() call from endpoint into MonitorValidator.create()
• Detector is created BEFORE alert_rule creation, allowing IssueAlertMigrator
  to find and link it to workflows
• Added skip_detector_creation context flag to prevent duplicate creation
  when using MonitorDataSourceValidator (new detector flow)
• Updated tests to verify detector creation

This ensures both the old (MonitorValidator) and new (detector API) flows
correctly create detectors for all monitors.

Part of NEW-593: Cron Monitor Alerts are not linked to monitor when created via the old flow
Original PR URL: getsentry#104004