test(workflow-operator): add ReservoirSamplingOpExec spec

[Ma77Ball]

What changes were proposed in this PR?

• Add ReservoirSamplingOpExecSpec, the first dedicated spec for the streaming reservoir-sampling executor.
• Cover processTuple buffering (emits nothing per tuple), and onFinish returning all input tuples in order when input size equals k.
• Cover keeping exactly k unique, input-drawn samples when input exceeds k, determinism of the seeded RNG (with a check that replacement actually happens), multi-worker partitioning of k via equallyPartitionGoal (k=10 across 3 workers yields 4,3,3), and open resetting state for executor reuse.
• Characterize the input-size-below-k edge case, where the unfilled fixed-size reservoir emits null padding on finish (flagged in the spec as a likely bug for a follow-up fix).

Any related issues, documentation, or discussions?

Closes: #5383

How was this PR tested?

• Run sbt "WorkflowOperator/testOnly *ReservoirSamplingOpExecSpec" and expect all 7 examples to pass.
• This is a test-only PR (no production code changed), so the spec itself is the verification; the input-below-k example intentionally asserts the current null-padding behavior, so a green run confirms that characterization rather than a fix.

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

Co-authored with Claude Opus 4.8 in compliance with ASF

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.45%. Comparing base (5c2eaa2) to head (a2bcf92).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5384      +/-   ##
============================================
- Coverage     51.61%   51.45%   -0.16%     
- Complexity     2448     2460      +12     
============================================
  Files          1067     1065       -2     
  Lines         41255    41203      -52     
  Branches       4437     4426      -11     
============================================
- Hits          21292    21202      -90     
- Misses        18712    18754      +42     
+ Partials       1251     1247       -4     

Flag	Coverage Δ		*Carryforward flag
access-control-service	42.22% <ø> (ø)
agent-service	33.76% <ø> (ø)		Carriedforward from 8aa81af
amber	52.29% <ø> (+0.13%)	⬆️
computing-unit-managing-service	1.65% <ø> (ø)
config-service	56.06% <ø> (ø)
file-service	38.32% <ø> (ø)
frontend	45.81% <ø> (-0.56%)	⬇️	Carriedforward from 8aa81af
pyamber	90.84% <ø> (+0.15%)	⬆️	Carriedforward from 8aa81af
python	90.84% <ø> (ø)		Carriedforward from 8aa81af
workflow-compiling-service	58.69% <ø> (ø)

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[Ma77Ball]

/request-review @aglinxinyuan

[xuang7]

Hi @Ma77Ball, we would like to backport this PR because #5606 depends on the tests introduced here. Could you open a backport PR targeting the release/v1.2 branch? Thank you!