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/custom_error_page.go

@@ -20,9 +20,7 @@ var CustomErrorPage = NewMiddleware[customErrorPage]()
 
 const StaticFilePathPrefix = "/$gperrorpage/"
 
-func (customErrorPage) requiresBodyRewrite() bool {
-	return true
-}
+func (customErrorPage) isBodyResponseModifier() {}
 
 // before implements RequestModifier.
 func (customErrorPage) before(w http.ResponseWriter, r *http.Request) (proceed bool) {