Add code coverage for DataGridViewComboBoxCell

[Zheng-Li01]

Related #13442

Proposed changes

Add unit test file:DataGridViewComboBoxCellTest.cs for public/protected methods of the DataGridViewComboBoxCell.cs

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

Attention: Patch coverage is 96.17486% with 7 lines in your changes missing coverage. Please review.

Project coverage is 76.71642%. Comparing base (9856a7c) to head (eeea645).
Report is 10 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #13654         +/-   ##
===================================================
+ Coverage   76.60680%   76.71642%   +0.10961%     
===================================================
  Files           3253        3256          +3     
  Lines         641011      641787        +776     
  Branches       47442       47502         +60     
===================================================
+ Hits          491058      492356       +1298     
+ Misses        146303      145768        -535     
- Partials        3650        3663         +13     

Flag	Coverage Δ
Debug	76.71642% <96.17486%> (+0.10961%)	⬆️
integration	18.98410% <ø> (-0.00346%)	⬇️
production	51.25056% <ø> (+0.18607%)	⬆️
test	97.41439% <96.17486%> (+0.00252%)	⬆️
unit	48.63198% <ø> (+0.18231%)	⬆️

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Pull Request Overview

This PR introduces a new unit test file to improve code coverage for the DataGridViewComboBoxCell component. The tests cover default values, property setters, exception throwing conditions, and behavior of key interactions and formatting methods.

• Added new unit tests for public and protected methods of DataGridViewComboBoxCell.
• Validated correct exception types and property behaviors.
• Ensured behavior consistency when changing properties under various conditions.

Comments suppressed due to low confidence (1)

src/test/unit/System.Windows.Forms/System/Windows/Forms/DataGridViewComboBoxCellTests.cs:7

• Consider refactoring to a file-scoped namespace definition (e.g. 'namespace System.Windows.Forms.Tests;') to simplify the code structure as recommended by modern C# practices.

namespace System.Windows.Forms.Tests;

[ricardobossan]

Thank you!