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/themed.go

@@ -54,6 +54,10 @@ func (m *themed) modifyResponse(resp *http.Response) error {
 	return m.m.modifyResponse(resp)
 }
 
+func (*themed) requiresBodyRewrite() bool {
+	return true
+}
+
 func (m *themed) finalize() error {
 	m.m.Target = "body"
 	if m.FontURL != "" && m.FontFamily != "" {