refactor(proxy): 统一代理拒绝请求生命周期处理

[g1331]

Summary

统一代理请求生命周期中的拒绝结果，确保上游调用前的拒绝请求保持现有 HTTP 契约，并正确释放队列、并发和取消资源。

Related Issue

Closes #260

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (code improvement without changing functionality)
• Tests (adding or updating tests)
• Build/CI (changes to build system or CI configuration)

Changes

• 将无效或过期 API key、权限不足、无匹配上游、配额/并发限制、排队超时和客户端取消统一纳入 lifecycle seam。
• 拒绝请求在上游 dispatch 前结束，不产生错误的成功计费或 recording 结果。
• 统一记录拒绝阶段、失败原因、耗时、路由决策和资源释放结果。
• 修正 queue admission、concurrency reservation、abort listener 与 failover reservation 的释放路径。
• 将下游 abort 传播到上游 fetch，并区分 dispatch 前后的取消语义。
• 增加基于 handleProxy 的生命周期行为测试。

Test Plan

• Local tests pass (pnpm test:run)
• Type check passes (pnpm exec tsc --noEmit)
• Lint passes (pnpm lint)
• Manual testing completed

Additional automated verification:

• pnpm test:proxy-stability passed, including production build and proxy stability smoke checks.
• pnpm format:check passed.

Checklist

• Code follows the project's coding standards
• Tests have been added where necessary
• Documentation has been updated (if applicable)
• Changes do not introduce security vulnerabilities
• Commit messages follow conventions

Screenshots

Not applicable; this PR has no UI changes.

Additional Notes

• Full suite result: 240 test files passed, 3588 tests passed, 1 skipped.
• Lint completed with 0 errors and 11 non-blocking jsdoc/require-jsdoc warnings.
• Production build completed successfully; Next.js emitted a Turbopack NFT tracing warning for the existing traffic-recording import path.

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.22%. Comparing base (9239970) to head (c9f6019).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #266      +/-   ##
==========================================
+ Coverage   72.21%   72.22%   +0.01%     
==========================================
  Files         244      244              
  Lines       14315    14344      +29     
  Branches     4764     4771       +7     
==========================================
+ Hits        10337    10360      +23     
- Misses       2609     2613       +4     
- Partials     1369     1371       +2     

Flag	Coverage Δ
verify	72.22% <86.66%> (+0.01%)	⬆️

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