feat: update Gemini adaptor to support custom response format

[mxdlzg]

我已确认该 PR 已自测通过，相关截图如下：
（此处放上测试通过的截图，如果不涉及前端改动或从 UI 上无法看出，请放终端启动成功的截图）

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 1.21%. Comparing base (fdd7bf4) to head (6e55a74).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
relay/adaptor/gemini/main.go	0.00%	8 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main   #1892      +/-   ##
========================================
- Coverage   1.27%   1.21%   -0.06%     
========================================
  Files        145     145              
  Lines      10379   12224    +1845     
========================================
+ Hits         132     149      +17     
- Misses     10233   12061    +1828     
  Partials      14      14              

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[songquanpeng]

Thx~