feat: normalize content coding boundaries

[tbphp]

Summary

Implements the confirmed GPT-Load 2.0 content-coding boundary:

• accept single-layer gzip, Brotli, deflate, and zstd client request bodies
• decode requests before dialect inspection and forwarding
• always send plaintext request bodies upstream with Accept-Encoding: identity
• keep compressed non-streaming upstream responses as a compatibility fallback, but return plaintext downstream
• reject compressed successful SSE before downstream commit
• reserve content-coding headers from group header rules
• return stable 400/406/413/415 local errors

Development state

This draft starts with the regression/contract tests only so CI demonstrates the expected RED state. The implementation commit will follow after the failing checks are confirmed.

Design source

Notion: GPT-Load 2.0 内容编码与压缩设计方案