test(amber): add unit tests for visualization OpDescs (HeatMap, BarChart, LineChart, PieChart)

[Yicong-Huang]

What changes were proposed in this PR?

Adds scalatest coverage for four more visualization operator descriptors. New specs for HeatMapOpDesc and LineChartOpDesc; existing thin BarChart and PieChart specs are extended with the same shape used in the previous bundle (#4809).

Each spec covers operatorInfo (name + group + outputPort count), getOutputSchemas (single-port html-content STRING), generatePythonCode (operator class + plotly imports + decode_python_template runtime decode-site count for each EncodableString), and the missing-required-field behavior — which differs by OpDesc:

• HeatMap asserts on x, y, value (all three).
• BarChart asserts on value and fields with explicit messages ("Value column cannot be empty" / "Fields cannot be empty").
• PieChart asserts only on value — name has no guard, so an empty-name configuration still renders.
• LineChart has no asserts but its lines field defaults to null; calling generatePythonCode on a default-constructed instance raises NullPointerException (see Bug filed below).

Any related issues, documentation, discussions?

Closes #4810.

Bug filed separately: LineChartOpDesc.generatePythonCode throws NullPointerException when lines is left at its null default, instead of raising AssertionError like the other visualizers or rendering an empty chart.

How was this PR tested?

sbt scalafmtCheckAll
sbt "WorkflowOperator/testOnly org.apache.texera.amber.operator.visualization.heatMap.HeatMapOpDescSpec org.apache.texera.amber.operator.visualization.barChart.BarChartOpDescSpec org.apache.texera.amber.operator.visualization.lineChart.LineChartOpDescSpec org.apache.texera.amber.operator.visualization.pieChart.PieChartOpDescSpec"

Was this PR authored or co-authored using generative AI tooling?

Generated-by: Claude Code (claude-opus-4-7)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.94%. Comparing base (9b2005d) to head (80038f1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #4812      +/-   ##
============================================
+ Coverage     43.85%   43.94%   +0.09%     
- Complexity     2106     2125      +19     
============================================
  Files           957      957              
  Lines         34072    34072              
  Branches       3753     3753              
============================================
+ Hits          14942    14974      +32     
+ Misses        18341    18309      -32     
  Partials        789      789              

Flag	Coverage Δ
access-control-service	28.12% <ø> (ø)
amber	42.58% <ø> (+0.20%)	⬆️
computing-unit-managing-service	0.00% <ø> (ø)
config-service	0.00% <ø> (ø)
file-service	33.24% <ø> (ø)
workflow-compiling-service	47.72% <ø> (ø)

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[Copilot]

Pull request overview

Adds ScalaTest unit coverage for additional visualization operator descriptors in common/workflow-operator, focusing on operatorInfo, getOutputSchemas, and structural properties of generatePythonCode, plus current “missing required field” behavior.

Changes:

• Added new specs for HeatMapOpDesc and LineChartOpDesc.
• Expanded existing BarChartOpDescSpec and PieChartOpDescSpec to validate operator metadata, output schema, python generation structure, and failure modes.
• Documented current asymmetric / buggy guard behavior (e.g., LineChartOpDesc NPE on default lines).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/visualization/pieChart/PieChartOpDescSpec.scala	Extends PieChart spec with operatorInfo/output schema/python code structural checks and guard behavior assertions.
common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/visualization/lineChart/LineChartOpDescSpec.scala	Adds new LineChart spec covering operatorInfo/output schema/python code structure and current null-lines NPE behavior.
common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/visualization/heatMap/HeatMapOpDescSpec.scala	Adds new HeatMap spec for operatorInfo/output schema/python code structure and required-field assertions.
common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/visualization/barChart/BarChartOpDescSpec.scala	Extends BarChart spec with operatorInfo/output schema/python code structural checks and assertion-message coverage.

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