test(amber): add unit tests for OrderingEnforcer

[Yicong-Huang]

What changes were proposed in this PR?

Adds scalatest coverage for amber/src/main/scala/org/apache/texera/amber/engine/architecture/messaginglayer/OrderingEnforcer.scala. The FIFO/exactly-once helper had no dedicated spec.

Any related issues, documentation, discussions?

Closes #4720.

How was this PR tested?

sbt scalafmtCheckAll
sbt "WorkflowExecutionService/testOnly org.apache.texera.amber.engine.architecture.messaginglayer.OrderingEnforcerSpec"

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 41.70%. Comparing base (0c56819) to head (a106a18).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #4721      +/-   ##
============================================
- Coverage     42.73%   41.70%   -1.04%     
- Complexity     2003     2015      +12     
============================================
  Files           957      978      +21     
  Lines         34077    34569     +492     
  Branches       3753     3779      +26     
============================================
- Hits          14564    14416     -148     
- Misses        18733    19373     +640     
  Partials        780      780              

Flag	Coverage Δ
access-control-service	?
amber	40.57% <ø> (+0.07%)	⬆️
computing-unit-managing-service	?
config-service	?
file-service	?
python	84.10% <ø> (-0.06%)	⬇️
workflow-compiling-service	?

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[Copilot]

Pull request overview

Adds a dedicated ScalaTest spec for Amber’s OrderingEnforcer (FIFO / exactly-once helper) to improve regression protection and close the existing coverage gap noted in #4720.

Changes:

• Introduces OrderingEnforcerSpec covering setCurrent, isDuplicated, isAhead, stash, and enforceFIFO behaviors.
• Exercises key FIFO draining scenarios: empty stash, contiguous drain, drain-until-gap, and non-contiguous stash behavior.

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