feat: support zstd request decompression

[seefs001]

⚠️ 提交说明 / PR Notice

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📝 变更描述 / Description

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支持对zstd进行解压

🚀 变更类型 / Type of change

• 🐛 Bug 修复 (Bug fix) - 请关联对应 Issue，避免将设计取舍、理解偏差或预期不一致直接归类为 bug
• ✨ 新功能 (New feature) - 重大特性建议先通过 Issue 沟通
• ⚡ 性能优化 / 重构 (Refactor)
• 📝 文档更新 (Documentation)

🔗 关联任务 / Related Issue

• Closes DecompressRequestMiddleware 缺少 zstd 分支：Content-Encoding: zstd 的 /v1/responses 请求体被当作裸 JSON 解析，报 invalid character '(' #6313 Codex returns 无效的请求 when model provider name is set to "OpenAI" #6537

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📸 运行证明 / Proof of Work

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Summary by CodeRabbit

• New Features

  • Added support for decoding Zstandard-compressed request bodies.
  • Improved handling of decompressed request headers for gzip, Brotli, and Zstandard content.

• Bug Fixes

  • Preserved request-size limits and error handling during decompression.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 03b28eea-ee3f-4693-83c7-6a976fe8aa3d

📥 Commits

Reviewing files that changed from the base of the PR and between 66ee6b8 and 2524b2c.

📒 Files selected for processing (2)

• go.mod
• middleware/gzip.go

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Walkthrough

The request decompression middleware now supports zstd alongside gzip and br, removes Content-Encoding after successful decoding, preserves body-size limiting, and closes decoder and source bodies correctly. The compression module is promoted to a direct dependency.

Changes

zstd request support

Layer / File(s)	Summary
Decoder dependency and wiring go.mod, middleware/gzip.go	The zstd package is imported, and github.com/klauspost/compress is declared as a direct requirement.
Request body decompression middleware/gzip.go	The middleware decodes zstd bodies, tracks decompression for supported encodings, clears the encoding header, limits decoded content, and handles reader failures and closure.

Estimated code review effort: 2 (Simple) | ~10 minutes

Sequence Diagram(s)

sequenceDiagram
  participant Client
  participant Middleware
  participant ZstdReader
  participant JSONHandler
  Client->>Middleware: Send zstd-encoded request
  Middleware->>ZstdReader: Create decoder
  ZstdReader-->>Middleware: Provide decompressed stream
  Middleware->>JSONHandler: Pass limited body without Content-Encoding

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Possibly related PRs

• QuantumNous/new-api#6348: Adds similar zstd request decompression and direct dependency promotion.

Suggested reviewers: calcium-ion

Poem

I’m a rabbit with compressed delight,
zstd now opens each request right.
Headers hop away, limits stay near,
Readers close cleanly without fear.
gzip, br, and zstd cheer!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely summarizes the main change: adding zstd request decompression support.
Linked Issues check	✅ Passed	The PR adds zstd handling to DecompressRequestMiddleware, removes the encoding header, and preserves error behavior as requested in #6313.
Out of Scope Changes check	✅ Passed	The dependency update in go.mod is directly needed for the new zstd decompression support and does not appear unrelated.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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