Flakiness in the test was identified (possibly) due to the use of static stateful fields in the @Extension class. See this comment for details. Issue was likely parallel test execution mutating the state of static field.

This PR addresses the issue following the comment here.

Note: This problem only affects tests and does not impact Jenkins functionality. Whether the data is stored in a static cache or within a field inside a singleton instance, it effectively centralizes event data in one place.

Testing done

• Automated tests still executing correctly.
• Manual testing
  • verify the duplicate events monitor functionality is correct as before (tested by sending duplicate events - admin monitor comes, restart jenkins - monitor goes away)

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue