add unit test coverage for the coordinator AdvanceRegionExecutionsHandler #7646

Task Summary

amber/src/main/scala/org/apache/texera/amber/engine/architecture/coordinator/promisehandlers/AdvanceRegionExecutionsHandler.scala (added in #6960) runs the region-execution advance in its own control round: handlers that need regions advanced send this RPC to the coordinator itself instead of advancing inline, so that terminating completed regions happens after every reply the requesting round owed. On failure it notifies the client with a FatalError, attaching the related worker id when the failure came from a worker RPC and None otherwise.

The handler body has no direct coverage: PortCompletedHandlerSpec asserts that the advance is requested (it checks the outgoing invocation) but never drives this handler, so the advance delegation and both FatalError notification paths are unasserted.

Task Type

• Refactor / Cleanup
• DevOps / Deployment / CI
• Testing / QA
• Documentation
• Performance
• Other