refactor(middleware): improve response body modification gating

Refactor response body modification to only allow text-like content types
(JSON, YAML, XML, etc.) instead of all HTML responses.

Body modification is now
blocked for binary content and transfer/content encoded responses, while status
code and headers can still be modified.

This prevents issues with compressed or streaming responses while
maintaining the ability to modify text-based API responses.

internal/net/gphttp/middleware/README.md

@@ -13,6 +13,8 @@ This package implements a flexible HTTP middleware system for GoDoxy. Middleware
 - **Bypass Rules**: Skip middleware based on request properties
 - **Dynamic Loading**: Load middleware definitions from files at runtime
 
+Response body rewriting is only applied to unencoded, text-like content types (for example `text/*`, JSON, YAML, XML). Response status and headers can always be modified.
+
 ## Architecture
 
 ```mermaid