[JENKINS-75586] Add configurable maximum concurrent indexing limit

[sinedd777]

• Add system property jenkins.branch.MultiBranchProject.maxConcurrentIndexing to configure the maximum
  number of concurrent indexing operations

  • Default value remains 5 to maintain backward compatibility
  • Override getCauseOfBlockage() to check against configurable limit
  • Add indexingCount() method to count actively indexing projects
  • Add user-friendly message when limit is reached

  Users can configure via:
  -Djenkins.branch.MultiBranchProject.maxConcurrentIndexing=

  Testing done

  • Manually tested with different values of the system property (3, 5, 10)
  • Verified that indexing is blocked when the limit is reached
  • Confirmed that the appropriate message is displayed when indexing is blocked
  • Tested that default behavior (limit of 5) is maintained when property is not set
  • Verified backward compatibility with existing configurations

  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 -
    JENKINS-75586
  • Link to relevant pull requests, esp. upstream and downstream changes
  • Ensure you have provided tests that demonstrate the feature works or the issue is fixed