style: optimize dropdown rtl style

[xrkffgg]

[中文版模板 / Chinese template]

🤔 This is a ...

• New feature
• Bug fix
• Site / documentation update
• Demo update
• Component style update
• TypeScript definition update
• Bundle size optimization
• Performance optimization
• Enhancement feature
• Internationalization
• Refactoring
• Code style optimization
• Test Case
• Branch merge
• Other (about what?)

🔗 Related issue link

💡 Background and solution

📝 Changelog

Language	Changelog
🇺🇸 English	Optimize Dropdown RTL writing to avoid dark mode style overlays.
🇨🇳 Chinese	优化 Dropdown RTL 写法避免暗黑模式样式覆盖。

☑️ Self Check before Merge

⚠️ Please check all items below before review. ⚠️

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• TypeScript definition is updated/provided or not needed
• Changelog is provided or not needed

[ant-design-bot]

[ant-design-bot]

[codesandbox-ci]

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[codecov]

Codecov Report

Merging #26206 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #26206   +/-   ##
=======================================
  Coverage   99.54%   99.54%           
=======================================
  Files         373      373           
  Lines        7315     7315           
  Branches     2040     1992   -48     
=======================================
  Hits         7282     7282           
  Misses         33       33           

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[github-actions]

Size Change: +9 B (0%)

Total Size: 803 kB

Filename	Size	Change
./dist/antd.compact.min.css	64.9 kB	+3 B (0%)
./dist/antd.dark.min.css	66.1 kB	+3 B (0%)
./dist/antd.min.css	64.9 kB	+3 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
./dist/antd-with-locales.min.js	319 kB	0 B
./dist/antd.min.js	288 kB	0 B

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