Add code coverage for TabRenderer

[Zheng-Li01]

Related #13442

Proposed changes
Add unit test file: TabRendererTests.cs for public methods of the TabRenderer.cs

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.59337%. Comparing base (5821493) to head (0fe109a).
Report is 88 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                  @@
##                main      #13482          +/-   ##
====================================================
+ Coverage   62.21083%   76.59337%   +14.38253%     
====================================================
  Files           3213        3243          +30     
  Lines         638302      640152        +1850     
  Branches       47201       47389         +188     
====================================================
+ Hits          397093      490314       +93221     
+ Misses        234153      146314       -87839     
+ Partials        7056        3524        -3532     

Flag	Coverage Δ
Debug	76.59337% <100.00000%> (+14.38253%)	⬆️
integration	18.50331% <ø> (-0.28548%)	⬇️
production	50.99944% <ø> (+32.21066%)	⬆️
test	97.41411% <100.00000%> (+0.00999%)	⬆️
unit	48.41134% <ø> (?)

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Pull Request Overview

Adds unit tests to verify that the TabRenderer APIs in WinForms behave as expected and do not throw exceptions when drawing tabs and pages.

• Introduces TabRendererTests for the IsSupported property
• Covers all DrawTabItem overloads (basic, with text, with image, combined)
• Adds a DrawTabPage test to confirm rendering changes pixel output

Comments suppressed due to low confidence (2)

src/test/unit/System.Windows.Forms/System/Windows/Forms/TabRendererTests.cs:4

• Missing using directives for Xunit and FluentAssertions; the test methods use [Fact], [WinFormsFact], Should(), and FluentAssertions APIs but these namespaces are not imported, which will cause compilation errors.

using System.Drawing;

src/test/unit/System.Windows.Forms/System/Windows/Forms/TabRendererTests.cs:15

• [nitpick] Consider guarding or skipping these rendering tests when visual styles are not enabled (TabRenderer.IsSupported == false), as drawing calls may become no-ops and result in flaky assertions on unsupported environments.

[WinFormsFact]

[ricardobossan]

Thank you!