perf: optimize request metadata extraction and disabled field filtering

[seefs001]

⚠️ 提交说明 / PR Notice

Important

• 请提供人工撰写的简洁摘要，避免直接粘贴未经整理的 AI 输出。

📝 变更描述 / Description

(简述：做了什么？为什么这样改能生效？请基于你对代码逻辑的理解来写，避免粘贴未经整理的内容)

• 轻量提取请求里的 model/group，避免分发阶段提前完整解析大请求体。
• 跳过无受控字段时的请求 JSON 重写，减少不必要的反序列化和重新序列化。
• 流式响应边接收边累计 usage 估算文本，避免保存全部 chunk 后再拼接重解析。

🚀 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 # (如有)

✅ 提交前检查项 / Checklist

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• Bug fix 说明: 若此 PR 标记为 Bug fix，我已提交或关联对应 Issue，且不会将设计取舍、预期不一致或理解偏差直接归类为 bug。
• 变更理解: 我已理解这些更改的工作原理及可能影响。
• 范围聚焦: 本 PR 未包含任何与当前任务无关的代码改动。
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📸 运行证明 / Proof of Work

(请在此粘贴截图、关键日志或测试报告，以证明变更生效)

Summary by CodeRabbit

Release Notes

• Performance

  • Optimized streaming response processing with incremental token handling
  • Improved request field validation efficiency

• Bug Fixes

  • Enhanced JSON request body parsing for reliable model selection

• Tests

  • Added comprehensive test coverage for request handling scenarios

[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

Run ID: caa5775e-77b6-469e-bd86-c787eb046c76

📥 Commits

Reviewing files that changed from the base of the PR and between 006e801 and ef617d5.

📒 Files selected for processing (5)

• middleware/distributor.go
• relay/channel/openai/helper.go
• relay/channel/openai/relay-openai.go
• relay/common/override_test.go
• relay/common/relay_info.go

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Walkthrough

This PR optimizes JSON request handling across the middleware and relay layers. It adds JSON body parsing for model selection in the distributor, refactors streaming token processing from batch buffering to incremental per-chunk parsing, and adds an early-return optimization for request field removal checks based on channel settings.

Changes

JSON and Stream Processing Improvements

Layer / File(s)	Summary
JSON Request Body Parsing for Model Selection middleware/distributor.go	getModelFromRequest now detects application/json content-type and delegates to new getModelFromJSONBody, which validates JSON, extracts model and group fields using gjson, enforces string types, and rewinds the body for downstream handlers. Adds io and gjson imports.
Stream Token Processing Optimization relay/channel/openai/helper.go, relay/channel/openai/relay-openai.go	Replaces batch-oriented stream helpers (processTokens, processChatCompletions, processCompletions) with incremental processTokenData that unmarshals each chunk on arrival. Removes streamItems buffering and post-stream processing; OaiStreamHandler now calls processTokenData inside the scan loop to accumulate into responseTextBuilder and toolCount.
Request Body Field Removal Optimization and Tests relay/common/relay_info.go, relay/common/override_test.go	Adds hasRemovableDisabledField helper using gjson to check existence of removable fields (service_tier, inference_geo, speed, store, safety_identifier, stream_options.include_obfuscation) against permission flags. RemoveDisabledFields now returns early without unmarshalling when no removable fields are present. Includes test asserting no mutation when no controlled fields exist.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~22 minutes

Poem

🐰 Hopping through JSON with gjson so fast,
Stream tokens no longer need buffering past,
Early returns save the day with a gleam,
Parsing and processing—a well-oiled team!
Bytes rewind smoothly, requests flow clean. 🌙

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 20.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title accurately summarizes the main changes: optimizing request metadata extraction and disabled field filtering across multiple files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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