NIFI-16248 - Merge clustered Connector purge request responses

[skeossei]

Summary

NIFI-16248

Merges asynchronous Connector purge responses across cluster nodes so the purge is reported as finished only after every node completes. This prevents Connector deletion from starting while another node still has queued FlowFiles.

Tracking

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Issue Tracking

• Apache NiFi Jira issue created

Pull Request Tracking

• Pull Request title starts with Apache NiFi Jira issue number, such as NIFI-00000
• Pull Request commit message starts with Apache NiFi Jira issue number, as such NIFI-00000
• Pull request contains commits signed with a registered key indicating Verified status

Pull Request Formatting

• Pull Request based on current revision of the main branch
• Pull Request refers to a feature branch with one commit containing changes

Verification

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Build

• Build completed using ./mvnw clean install -P contrib-check
  • JDK 21
  • JDK 25

Focused verification completed using JDK 21:

• ConnectorPurgeRequestEndpointMergerTest, StandardHttpResponseMapperTest, and DropRequestEndpointMergerTest: 10 tests passed
• ClusteredConnectorBacklogIT: 3 tests passed
• ClusteredConnectorBacklogIT#testClusteredReadQueueAttributesLoadBalancedConnection: 5 consecutive runs passed
• Targeted nifi-framework-cluster contribution checks passed, including Checkstyle, Apache RAT, and PMD

Licensing

• New dependencies are compatible with the Apache License 2.0 according to the License Policy
• New dependencies are documented in applicable LICENSE and NOTICE files

No new dependencies were added, so no LICENSE or NOTICE changes are required.

Documentation

• Documentation formatting appears as expected in rendered files

No documentation files were changed.

[exceptionfactory]

Thanks for addressing this issue @skeossei. The basic addition of the Merger class looks good, I highlighted a handful of stylistic recommendations