fix(middleware): gate only body response modifiers

Replace the rewrite requirement check with a BodyResponseModifier
marker and treat header and body modifiers separately.

This ensures header/status rewrites still apply when body rewriting is
blocked (for binary, encoded, or chunked responses), while body changes
are skipped safely. It also avoids body reset/close side effects and
returns early on passthrough responses.

Update middleware tests to cover split header/body behavior and themed
middleware body-skip scenarios.

internal/net/gphttp/middleware/modify_html.go

@@ -22,9 +22,7 @@ type modifyHTML struct {
 
 var ModifyHTML = NewMiddleware[modifyHTML]()
 
-func (*modifyHTML) requiresBodyRewrite() bool {
-	return true
-}
+func (*modifyHTML) isBodyResponseModifier() {}
 
 func (m *modifyHTML) before(_ http.ResponseWriter, req *http.Request) bool {
 	req.Header.Set("Accept-Encoding", "identity")