fix(gateway): 429 同 key 只再试一次，换 key 前拉长退避

[xiaocheny214]

Summary

• 同一把 key 遇到 429 只再试一次（共 2 次），不再连打三次。
• 仍 429 则等 16s（同 key 再试 8s；指数退避上限 60s；Retry-After 作下限）再切下一把 key；新 key 同样只再试一次。
• 本入口 key 用尽则熔断该 base_url，切备用入口；没有备用入口则熔断 aggregator。
• 图 / 视频 / chat 三条 Gateway 共用同一套策略。52x、空响应不变。

Closes #817

Test plan

• uv run pytest tests/test_gateway_policy.py tests/test_gateway_image.py tests/test_gateway_video.py tests/test_gateway_chat.py -q
• 单 key：2 次后失败并打开 aggregator 熔断
• 多 key：key-a 两次后等 8s+16s 切 key-b
• 本入口 key 用尽后切 backup，并熔断 primary
• 52x UNREACHED 仍只同 key 再试一次，不走这条退避

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
windup	Ignored	Preview	Aug 27, 2026 8:42am

[codecov]

Codecov Report

❌ Patch coverage is 85.00000% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ges/framework/src/windup_framework/gateway/chat.py	66.66%	3 Missing ⚠️
...es/framework/src/windup_framework/gateway/video.py	78.57%	3 Missing ⚠️

@@            Coverage Diff             @@
##             main     #818      +/-   ##
==========================================
- Coverage   87.09%   86.51%   -0.59%     
==========================================
  Files         288      293       +5     
  Lines       21559    21918     +359     
  Branches     3299     3329      +30     
==========================================
+ Hits        18777    18962     +185     
- Misses       1852     2026     +174     
  Partials      930      930              

Flag	Coverage Δ
backend	91.51% <85.00%> (-0.24%)	⬇️
frontend	80.54% <ø> (-1.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...es/framework/src/windup_framework/gateway/image.py	91.62% <100.00%> (+0.13%)	⬆️
...s/framework/src/windup_framework/gateway/policy.py	100.00% <100.00%> (ø)
...ges/framework/src/windup_framework/gateway/chat.py	89.41% <66.66%> (-0.93%)	⬇️
...es/framework/src/windup_framework/gateway/video.py	82.52% <78.57%> (-1.02%)	⬇️

... and 26 files with indirect coverage changes

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[fennoai]

审阅了共享退避策略以及 chat、image、video 三条 Gateway 的 429 重试、换 key 和非 429 重试路径。改动符合 #817 的契约：同一 key 仍执行三次调用，等待为 2s/4s，Retry-After 作为下限并保留 30s 封顶；UNREACHED 等其他策略未被改动。

未发现达到报告阈值的 correctness、可靠性或兼容性问题。验证方面，compileall 与 git diff --check 通过；PR 描述中的 pytest 命令未能执行，因为当前环境没有 uv 或 pytest。

[johnnyzhang-eng]

LGTM