Validate full git refs provided for gitlab_pipeline_schedule

Summary

This change adds a validation rule to the ref field of pipeline schedules, ensuring that users must provide a full Git reference (e.g., refs/heads/main or refs/tags/v1.0.0) rather than a short branch or tag name (e.g., main or v1.0.0). If an invalid value is provided, a clear error message is shown. A new test is also added to confirm that short branch names and tag names are correctly rejected.

Fixes #6449 (closed)