add unit test coverage for ComputingUnitManagingServiceException

[mengw15]

Task Summary

computing-unit-managing-service/src/main/scala/org/apache/texera/service/util/ComputingUnitManagingServiceException.scala (0% coverage) defines a sealed exception hierarchy plus fromKubernetes, which classifies a KubernetesClientException into a typed exception. Pure logic, no DB — a plain AnyFlatSpec covers it.

Behavior to cover

• InsufficientComputingResource("CPU").getMessage == "Insufficient CPU available in the server. Please decrease the requested amount or try again later."
• InsufficientComputingUnitQuota(3).getMessage == "You may only have 3 computing-unit(s) running at the same time"
• InternalError().getMessage == the default "The server encountered an internal error while processing your request. Please try again later."
• fromKubernetes — message contains "exceeded quota" and "cpu" -> InsufficientComputingResource("CPU"); "memory" -> ("memory"); "gpu" -> ("GPU")
• fromKubernetes — "exceeded quota" but none of cpu/memory/gpu -> InternalError(e.getMessage)
• fromKubernetes — message without "exceeded quota" -> InternalError(e.getMessage)
• fromKubernetes — null message -> InternalError (the Option(e.getMessage).getOrElse("") path)
• Case-insensitivity: "Exceeded Quota: CPU limit" classifies as CPU (message is lower-cased before matching)

Task Type

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