fix: paste rich text with br should keep line break

[ybzky]

close #4454

Pull Request Checklist

• Related tickets or issues have been linked in the PR description (or missing issue).
• Naming convention is followed (do please check it especially when you created new plugins, commands and resources).
• Unit tests have been added for the changes (if applicable).
• Breaking changes have been documented (or no breaking changes introduced in this PR).

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Playwright test results

 24 passed

Details

 24 tests across 11 suites
 5 minutes, 33 seconds
 6984d9f
 For more information, see full report

[codecov]

Codecov Report

Attention: Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 32.66%. Comparing base (95f6b9d) to head (6984d9f).
Report is 4 commits behind head on dev.

Files with missing lines	Patch %	Lines
...ui/src/services/clipboard/html-to-usm/converter.ts	20.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #4670      +/-   ##
==========================================
+ Coverage   32.64%   32.66%   +0.01%     
==========================================
  Files        2606     2606              
  Lines      135670   135675       +5     
  Branches    30204    30206       +2     
==========================================
+ Hits        44296    44318      +22     
+ Misses      91374    91357      -17     

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[yuhongz]

LGTM