Add unit test coverage for operator utility classes (OperatorDescriptorUtils, DistributedAggregation, URLFetchUtil)

[aglinxinyuan]

Task Summary

Three small utility classes/objects under common/workflow-operator/ have no <srcClassName>Spec.scala. They underpin numerical-partition behavior across worker counts (OperatorDescriptorUtils), the distributed-aggregate contract every aggregator obeys (DistributedAggregation), and the user-facing URL-fetch retry path (URLFetchUtil).

Scope

Source class	Lines	What's currently un-pinned
common/workflow-operator/.../util/OperatorDescriptorUtils.scala	52	equallyPartitionGoal(goal, workers) sums to goal; ranges where goal < workers, goal == workers, goal > workers; remainder lands on the first goal % workers slots; toImmutableMap round-trip
common/workflow-operator/.../aggregate/DistributedAggregation.scala	52	case-class equality + per-field accessors; end-to-end init→iterate→merge→finalAgg worked example (running average)
common/workflow-operator/.../source/fetcher/URLFetchUtil.scala	43	success path returns Some(stream); failure path retries N times and returns None; default retries == 5

Out of scope

No production-code changes. One <srcClassName>Spec.scala per source class.

Task Type

• Testing / QA