fix: replace fixed sleep with polling in notification test assertions

Summary

This MR fixes the flaky test TestGitlabAPI_RenameRepository_WithBaseRepository/dry_run_param_not_set_means_implicit_false by replacing the fixed 50ms time.Sleep() with a polling-based approach using require.Eventually().

The test was failing intermittently due to a race condition between asynchronous webhook notification delivery and the test's event assertion logic. The hardcoded 50ms delay was insufficient in database load balancing scenarios where:

• WAL replication delays occur between primary and replicas
• Service discovery DNS resolution adds latency
• Connection pool management introduces overhead
• CI environment resources vary unpredictably

This MR replaces the fixed delay with require.Eventually().

Closes #1787 (closed)

Checklist

• Code follows repository patterns and conventions
• All functions are fully implemented with no TODOs
• Proper error handling and validation
• Comments added for key functions
• Conventional commit message format