fix: cdc merge union schema order

[ion-elgreco]

Description

DataFusion doesn't have unionByName or a relaxed concat, so the before and after schema needs to be in the same order. The before though was getting expressions in different order. So just doing a simple select_columns afterwards to take the after schema order (which has the write projection order)

Related Issue(s)

• closes Table merges fail when CDF is enabled due to column mismatches  #2908
• closes Error with MERGE for partitioned table having delta.enableChangeDataFeed set to true and non-primitive columns #2832

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 72.48%. Comparing base (72e344e) to head (c6ba0c8).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/core/src/operations/merge/mod.rs	75.00%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2919      +/-   ##
==========================================
+ Coverage   72.47%   72.48%   +0.01%     
==========================================
  Files         131      131              
  Lines       40410    40418       +8     
  Branches    40410    40418       +8     
==========================================
+ Hits        29287    29299      +12     
  Misses       9238     9238              
+ Partials     1885     1881       -4     

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