Add code coverage for DataGridViewImageCell

[Zheng-Li01]

Related #13442

Proposed changes

Add unit test file:DataGridViewImageCellTests.cs for public/protected methods/properties of the DataGridViewImageCell.cs

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.61283%. Comparing base (ece6f6d) to head (1a6746a).

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #13616         +/-   ##
===================================================
+ Coverage   76.58723%   76.61283%   +0.02559%     
===================================================
  Files           3253        3254          +1     
  Lines         640992      641155        +163     
  Branches       47439       47450         +11     
===================================================
+ Hits          490918      491207        +289     
+ Misses        146541      146391        -150     
- Partials        3533        3557         +24     

Flag	Coverage Δ
Debug	76.61283% <100.00000%> (+0.02559%)	⬆️
integration	18.47240% <ø> (-0.00976%)	⬇️
production	51.06554% <ø> (+0.04379%)	⬆️
test	97.41294% <100.00000%> (+0.00119%)	⬆️
unit	48.51603% <ø> (+0.08409%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[Copilot]

Pull Request Overview

This PR introduces a new suite of unit tests for DataGridViewImageCell, covering its constructors, property getters, layout calculations, painting logic, and ToString behavior.

• Adds DataGridViewImageCellTests.cs with tests for default ctor behavior, DefaultNewRowValue, content/error bounds, preferred size, painting, and string representation.
• Uses the test accessor to exercise protected layout and rendering methods.
• Verifies exception behavior and default values under various DataGridView configurations.

Comments suppressed due to low confidence (2)

src/test/unit/System.Windows.Forms/System/Windows/Forms/DataGridViewImageCellTests.cs:10

• [nitpick] Property names in C# should use PascalCase. Consider renaming dataGridViewImageCell to Cell or DataGridViewImageCellInstance.

    public DataGridViewImageCell dataGridViewImageCell => new();

src/test/unit/System.Windows.Forms/System/Windows/Forms/DataGridViewImageCellTests.cs:95

• All GetContentBounds tests cover only the empty‐bounds paths. To improve coverage, add a test where OwningColumn and a valid image value are set so that GetContentBounds returns a non‐empty rectangle.

        Rectangle bounds = (Rectangle)dataGridViewImageCell.TestAccessor().Dynamic.GetContentBounds(g, style, 0);