fix: Remove the Authorization request header when using AI-proxy to proxy Gemini

[hanxiantao]

Ⅰ. Describe what this PR did

ai-proxy 代理 gemini 时移除 Authorization 请求头

Ⅱ. Does this pull request fix one issue?

fixes #2199

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

修复前原始请求带 Authorization 请求头代理 gemini 时，报错：

修复后原始请求带 Authorization 请求头也能正常代理到 gemini

Ⅴ. Special notes for reviews

[CH3CHO]

LGTM

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.82%. Comparing base (ef31e09) to head (e3c68b6).
Report is 504 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2220      +/-   ##
==========================================
+ Coverage   35.91%   45.82%   +9.91%     
==========================================
  Files          69       79      +10     
  Lines       11576    12912    +1336     
==========================================
+ Hits         4157     5917    +1760     
+ Misses       7104     6649     -455     
- Partials      315      346      +31     

see 77 files with indirect coverage changes

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