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/custom_error_page.go

@@ -20,6 +20,10 @@ var CustomErrorPage = NewMiddleware[customErrorPage]()
 
 const StaticFilePathPrefix = "/$gperrorpage/"
 
+func (customErrorPage) requiresBodyRewrite() bool {
+	return true
+}
+
 // before implements RequestModifier.
 func (customErrorPage) before(w http.ResponseWriter, r *http.Request) (proceed bool) {
 	return !ServeStaticErrorPageFile(w, r)