[IcebergIO] add test for column default values

[ahmedabu98]

Default column values is an Iceberg V3 feature: https://iceberg.apache.org/spec/#version-3-extended-types-and-capabilities

Add tests to verify that we support it (since we're using the latest Iceberg libraries)

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[Copilot]

Pull request overview

This PR adds integration test coverage for Iceberg V3 column default values in the Iceberg CDC incremental changelog reader, ensuring Beam’s IcebergIO correctly materializes defaulted values when reading older files after schema evolution.

Changes:

• Add a test verifying that an older data file (written before schema evolution) is read with the new column’s initial default value.
• Add a test verifying overwrite CDC semantics where the “before” row uses the initial default (old file) and the “after” row uses an explicitly written value (new file).
• Add a small formatting DoFn used to assert defaulted vs explicit values in formatted CDC output.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.17%. Comparing base (a0e2714) to head (7e680bf).

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #39739      +/-   ##
============================================
+ Coverage     54.97%   58.17%   +3.20%     
- Complexity     1722    13104   +11382     
============================================
  Files          1071     2523    +1452     
  Lines        170836   265755   +94919     
  Branches       1262    10795    +9533     
============================================
+ Hits          93910   154600   +60690     
- Misses        74656   105336   +30680     
- Partials       2270     5819    +3549     

Flag	Coverage Δ
java	64.27% <ø> (-3.41%)	⬇️

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