Add code coverage for DataGridViewComboBoxColumn

[Zheng-Li01]

Related https://github.com/dotnet/winforms/issues/13442

Proposed changes

Add unit test file:DataGridViewComboBoxColumnTest.cs for public methods of the DataGridViewComboBoxColumn.cs

Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 99.65753% with 1 line in your changes missing coverage. Please review.

Project coverage is 76.83288%. Comparing base (ece6f6d) to head (617496c). Report is 50 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #13617         +/-   ##
===================================================
+ Coverage   76.58723%   76.83288%   +0.24564%     
===================================================
  Files           3253        3257          +4     
  Lines         640992      642471       +1479     
  Branches       47439       47572        +133     
===================================================
+ Hits          490918      493629       +2711     
+ Misses        146541      145182       -1359     
- Partials        3533        3660        +127     

Flag	Coverage Δ
Debug	76.83288% <99.65753%> (+0.24564%)	:arrow_up:
integration	18.99490% <ø> (+0.51274%)	:arrow_up:
production	51.45490% <ø> (+0.43315%)	:arrow_up:
test	97.41965% <99.65753%> (+0.00790%)	:arrow_up:
unit	48.83145% <ø> (+0.39951%)	:arrow_up:

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