Add code coverage for RichTextBox

[Zheng-Li01]

related https://github.com/dotnet/winforms/issues/10453

Proposed changes

• Add unit tests for RichTextBox to test its DrawToBitmap()/LoadFile()/SaveFile()/Paste() methods

Customer Impact

• None

Regression?

• No

Risk

• Minimal

Test methodology

• Unit tests

Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.97980% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 73.23597%. Comparing base (5e635c5) to head (f82ba91). Report is 34 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #10994         +/-   ##
===================================================
+ Coverage   73.19881%   73.23597%   +0.03716%     
===================================================
  Files           3097        3096          -1     
  Lines         634289      633817        -472     
  Branches       47364       47338         -26     
===================================================
- Hits          464292      464182        -110     
+ Misses        166453      166089        -364     
- Partials        3544        3546          +2     

Flag	Coverage Δ
Debug	73.23597% <97.97980%> (+0.03716%)	:arrow_up:
integration	18.42168% <ø> (-0.02558%)	:arrow_down:
production	46.72061% <ø> (+0.01158%)	:arrow_up:
test	94.98148% <97.97980%> (+0.01015%)	:arrow_up:
unit	43.60598% <ø> (-0.00078%)	:arrow_down:

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

[ricardobossan]

LGTM