add unit test coverage for storage result + dataset model classes

[aglinxinyuan]

Task Summary

Add dedicated unit-specs for three small storage-layer model classes in common/workflow-core/. Pin defaults, equality, and trait conformance so accidental drift in the result-store schema (tupleCount, changeDetector) or in the OnDataset accessor surface is caught early.

Background

Three small storage-layer model classes in common/workflow-core currently lack a dedicated unit-spec:

Source class	Package	Purpose
OperatorResultMetadata	core/storage/result	Per-operator result metadata: tupleCount: Int = 0, changeDetector: String = ""
WorkflowResultStore	core/storage/result	Map-wrapping case class: resultInfo: Map[OperatorIdentity, OperatorResultMetadata] = Map.empty
OnDataset	core/storage/model	Trait with getRepositoryName / getVersionHash / getFileRelativePath — accessor contract for dataset-backed documents

Behavior to pin

OperatorResultMetadata

Surface	Contract
Default constructor	OperatorResultMetadata() has tupleCount == 0 and changeDetector == ""
Custom values	preserved via the case-class constructor
Equality	case-class equality compares both fields
copy	replaces only the field passed, preserves the rest

WorkflowResultStore

Surface	Contract
Default constructor	WorkflowResultStore() has resultInfo == Map.empty
Equality	two stores with the same resultInfo are case-equal
copy	replaces resultInfo
Map carrying multiple operator metadatas	each key/value is preserved

OnDataset

Surface	Contract
Trait declares three accessors	getRepositoryName, getVersionHash, getFileRelativePath — all (): String
A test-only concrete subclass	returns each accessor's value verbatim
Pattern matching	a value of type OnDataset can be matched via type-test for cross-module dispatch

Scope

• New spec files (one per source class per the spec-filename convention):
  • OperatorResultMetadataSpec.scala
  • WorkflowResultStoreSpec.scala
  • OnDatasetSpec.scala
• No production-code changes.
• OnDataset is a trait; exercise it through a minimal test-only concrete subclass.

Task Type

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