Add code coverage for RichTextBox

[Zheng-Li01]

Fixes #10453

Proposed changes

• Add unit tests for RichTextBox to test its DrawToBitmap()/LoadFile()/SaveFile()/Paste() methods
• Add unit tests for RichTextBoxto test its events: DragDrop, DragEnter, DragLeave, DragOver, GiveFeedback,QueryContinueDrag

Customer Impact

• None

Regression?

• No

Risk

• Minimal

Test methodology

• Unit tests

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

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

Project coverage is 73.23675%. Comparing base (5e635c5) to head (fbc7a22).
Report is 33 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #11034         +/-   ##
===================================================
+ Coverage   73.19881%   73.23675%   +0.03794%     
===================================================
  Files           3097        3096          -1     
  Lines         634289      633817        -472     
  Branches       47364       47338         -26     
===================================================
- Hits          464292      464187        -105     
+ Misses        166453      166090        -363     
+ Partials        3544        3540          -4     

Flag	Coverage Δ
Debug	73.23675% <97.97980%> (+0.03794%)	⬆️
integration	18.42765% <ø> (-0.01961%)	⬇️
production	46.72236% <ø> (+0.01333%)	⬆️
test	94.98148% <97.97980%> (+0.01015%)	⬆️
unit	43.62874% <ø> (+0.02197%)	⬆️

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