fix: drop withDefaultValue from StatisticsManager so checkpoint state round-trips

[Ma77Ball]

What changes were proposed in this PR?

StatisticsManager declared its input/output stats maps as mutable.Map.empty.withDefaultValue((0L, 0L)). The resulting Map.WithDefault wrapper does not survive a Kryo round-trip (its inner map deserializes as null), so
chkpt.load(CP_STATE_KEY) on a default-state ControllerProcessor throws KryoException: NullPointerException, blocking Controller.loadFromCheckpoint from ever rehydrating a checkpointed controller. This PR removes the wrapper and inlines getOrElse(portId, (0L, 0L)) at the two write sites; behavior is unchanged.

Any related issues, documentation, or discussions?

closes: #4686

How was this PR tested?

Replaced the two existing should be serializable cases in CheckpointSpec with full save then load round-trips (controller + worker) that assert restored.actorId == original.actorId; the new tests reproduce the original NPE on main and pass after the fix. Verified locally with sbt 'WorkflowExecutionService / Test / testOnly org.apache.texera.amber.engine.faulttolerance.CheckpointSpec' (3/3 pass).

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

Co-authored with Claude Opus 4.7 in compliance with ASF

[Ma77Ball]

/request-review @aglinxinyuan

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.35%. Comparing base (bf2f92c) to head (c1870e3).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #5150   +/-   ##
=========================================
  Coverage     43.35%   43.35%           
  Complexity     2212     2212           
=========================================
  Files          1049     1049           
  Lines         40560    40558    -2     
  Branches       4322     4320    -2     
=========================================
  Hits          17583    17583           
  Misses        21886    21886           
+ Partials       1091     1089    -2     

Flag	Coverage Δ		*Carryforward flag
access-control-service	39.53% <ø> (ø)		Carriedforward from 07be263
agent-service	33.76% <ø> (ø)		Carriedforward from 07be263
amber	43.81% <100.00%> (+<0.01%)	⬆️
computing-unit-managing-service	0.00% <ø> (ø)		Carriedforward from 07be263
config-service	0.00% <ø> (ø)		Carriedforward from 07be263
file-service	32.18% <ø> (ø)		Carriedforward from 07be263
frontend	34.61% <ø> (ø)		Carriedforward from 07be263
python	90.50% <ø> (ø)		Carriedforward from 07be263
workflow-compiling-service	56.81% <ø> (ø)		Carriedforward from 07be263

*This pull request uses carry forward flags. Click here to find out more.

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