feat: add stats transform module for checkpoint stats population

[DrakeLin]

🥞 Stacked PR

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• stack/stats-schema [Files changed]
  • stack/null-propagation [Files changed]
    • stack/coalesce [Files changed]
      • stack/checkpoint-transforms [Files changed]
        • stack/write-stats [Files changed]

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What changes are proposed in this pull request?

Adds a new stats_transform module that provides the core logic for populating stats and stats_parsed fields when writing checkpoints. This module builds transform expressions based on table configuration to ensure statistics are properly converted between JSON and struct formats.

writeStatsAsJson	writeStatsAsStruct	stats	stats_parsed
true	false	COALESCE(stats, ToJson(stats_parsed))	drop
true	true	COALESCE(stats, ToJson(stats_parsed))	COALESCE(stats_parsed, ParseJson(stats))
false	true	drop	COALESCE(stats_parsed, ParseJson(stats))
false	false	drop	drop

How was this change tested?

Next Pr

[codecov]

Codecov Report

❌ Patch coverage is 83.12883% with 55 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.11%. Comparing base (435b94e) to head (eab701e).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
kernel/src/checkpoint/stats_transform.rs	78.04%	54 Missing ⚠️
...src/engine/arrow_expression/evaluate_expression.rs	94.44%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1646      +/-   ##
==========================================
- Coverage   84.15%   84.11%   -0.04%     
==========================================
  Files         123      124       +1     
  Lines       34180    34498     +318     
  Branches    34180    34498     +318     
==========================================
+ Hits        28764    29019     +255     
- Misses       4021     4070      +49     
- Partials     1395     1409      +14     

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