fix: mixed policy in rendering cells in bg extension

[lumixraku]

close https://github.com/dream-num/univer-pro/issues/3280

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

 19 passed

Details

 19 tests across 9 suites
 4 minutes, 38 seconds
 bd11940
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Codecov Report

Attention: Patch coverage is 0% with 21 lines in your changes missing coverage. Please review.

Project coverage is 32.92%. Comparing base (54599ce) to head (bd11940).
Report is 21 commits behind head on dev.

Files with missing lines	Patch %	Lines
...der/src/components/sheets/extensions/background.ts	0.00%	21 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #4048      +/-   ##
==========================================
+ Coverage   32.86%   32.92%   +0.06%     
==========================================
  Files        2501     2497       -4     
  Lines      126591   126913     +322     
  Branches    28351    28416      +65     
==========================================
+ Hits        41598    41792     +194     
- Misses      84993    85121     +128     

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[wzhudev]

Code duplication.