refactor: add Expression::coalesce helper method

[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?

This PR adds an Expression::coalesce helper method to simplify creating COALESCE expressions, and updates all existing call sites to use the new helper.

How was this change tested?

Existing tests

[codecov]

Codecov Report

❌ Patch coverage is 98.75000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 84.16%. Comparing base (435b94e) to head (287577f).
⚠️ Report is 2 commits behind head on main.

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

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1648   +/-   ##
=======================================
  Coverage   84.15%   84.16%           
=======================================
  Files         123      123           
  Lines       34180    34252   +72     
  Branches    34180    34252   +72     
=======================================
+ Hits        28764    28827   +63     
+ Misses       4021     4016    -5     
- Partials     1395     1409   +14     

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