add unit test coverage for visualization chart-config classes

[aglinxinyuan]

Task Summary

Add dedicated unit-specs for four small Jackson-annotated config classes that back visualization operators in common/workflow-operator/operator/visualization/. Pin defaults, mutability, JSON serde, and required-field validation so accidental drift in a Jackson annotation (or default value) is caught before it ships to a chart that depends on it.

Background

Four config classes in common/workflow-operator/operator/visualization/ currently lack a dedicated unit-spec. Each is a small Jackson-serializable bag carrying a single attribute name (sometimes two) used by the corresponding visualization operator:

Source class	Package	Fields
TablesConfig	tablesChart	attributeName: EncodableString = "" (required, @NotNull)
NestedTableConfig	nestedTable	attributeGroup, originalName (required), newName (optional)
FigureFactoryTableConfig	figureFactoryTable	attributeName: EncodableString = "" (required)
DumbbellDotConfig	dumbbellPlot	dotValue: EncodableString = "" (required, @NotNull, integer/long/double-only via @JsonSchemaInject)

Behavior to pin

For each config class:

Surface	Contract
Default field values	the documented defaults ("", etc.) hold on a fresh instance
Mutability	fields are var and can be assigned post-construction (mutability is part of the Jackson-bag contract — readers need this)
JSON round-trip via objectMapper.writeValueAsString + readValue	preserves every field
@JsonProperty(required = true) annotation	present on the documented required field(s) — verified via reflection
Distinct instances	two new TablesConfig (etc.) are independent (no static-field leakage)
@NotNull annotation	present on the documented @NotNull field(s) — verified via reflection

Scope

• New spec files (one per source class per the spec-filename convention):
  • TablesConfigSpec.scala
  • NestedTableConfigSpec.scala
  • FigureFactoryTableConfigSpec.scala
  • DumbbellDotConfigSpec.scala
• No production-code changes.
• The Jackson serde tests use org.apache.texera.amber.util.JSONUtils.objectMapper (already wired with the project's modules).

Task Type

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