fix(middleware): skip body rewriters when buffering fails

Prevent response modifiers that require body rewriting from running when
the body rewrite gate blocks buffering (for example, chunked transfer
encoding).

Add an explicit `requiresBodyRewrite` capability and implement it for
HTML/theme/error-page modifiers, including bypass delegation.

Also add a regression test to ensure the original response body remains
readable and is not closed prematurely when rewrite is blocked.

This commit fixeds the "http: read on closed response body" with empty page error
happens when body-rewriting middleware (like themed) runs on responses where body rewrite is blocked (e.g. chunked),
then the gate restores an already-closed original body.

internal/net/gphttp/middleware/modify_html.go

@@ -22,6 +22,10 @@ type modifyHTML struct {
 
 var ModifyHTML = NewMiddleware[modifyHTML]()
 
+func (*modifyHTML) requiresBodyRewrite() bool {
+	return true
+}
+
 func (m *modifyHTML) before(_ http.ResponseWriter, req *http.Request) bool {
 	req.Header.Set("Accept-Encoding", "identity")
 	return true