feat(ai-proxy): add doubao Image Generation support

[daixijun]

Ⅰ. Describe what this PR did

Ⅱ. Does this pull request fix one issue?

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

Ⅳ. Describe how to verify it

curl -v "http://localhost:10000/v1/images/generations" \
  -H "Content-Type: application/json" \
  -d '{
        "model": "doubao-seedream-3-0-t2i-250415",
        "prompt": "a portrait of a sheepadoodle wearing a cape",
        "response_format": "url",
        "n": 1, 
        "size": "1024x1024"      
      }'

{"model":"doubao-seedream-3-0-t2i-250415","created":1748342039,"data":[{"url":"https://ark-content-generation-v2-cn-beijing.tos-cn-beijing.volces.com/doubao-seedream-3-0-t2i/02174834203602939715a70a10229992a5710e25d9af35e611ec8.jpeg?X-Tos-Algorithm=TOS4-HMAC-SHA256&X-Tos-Credential=xxxxx%2F20250527%2Fcn-beijing%2Ftos%2Frequest&X-Tos-Date=20250527T103359Z&X-Tos-Expires=86400&X-Tos-Signature=fc040a66aa86d5a906d1e76a13e3d9eb461facfdb141834de7f20ee745e3d945&X-Tos-SignedHeaders=host&x-tos-process=image%2Fwatermark%2Cimage_YXNzZXRzL3dhdGVybWFyay5wbmc_eC10b3MtcHJvY2Vzcz1pbWFnZS9yZXNpemUsUF8xNg%3D%3D"}],"usage":{"generated_images":1}}

Ⅴ. Special notes for reviews

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 46.08%. Comparing base (ef31e09) to head (0797f1c).
Report is 524 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2331       +/-   ##
===========================================
+ Coverage   35.91%   46.08%   +10.17%     
===========================================
  Files          69       81       +12     
  Lines       11576    13010     +1434     
===========================================
+ Hits         4157     5996     +1839     
+ Misses       7104     6669      -435     
- Partials      315      345       +30     

see 78 files with indirect coverage changes

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[CH3CHO]

LGTM