fix(middleware): stream header-only response rewrites without body buffering

Header-only modifiers no longer use LazyResponseModifier with buffering
always enabled. They wrap the response with ModifyResponseWriter and
return after invoking the next handler.

Body modifiers still use LazyResponseModifier with canBufferAndModifyResponseBody.
Add a regression test that uses a 64MiB Content-Length with a small body.

internal/net/gphttp/middleware/middleware.go

@@ -213,14 +213,13 @@ func (m *Middleware) ServeHTTP(next http.HandlerFunc, w http.ResponseWriter, r *
 		return
 	}
 	isBodyModifier := isBodyResponseModifier(exec)
-
-	shouldBuffer := canBufferAndModifyResponseBody
 	if !isBodyModifier {
-		// Header-only response modifiers do not need body rewrite capability checks.
-		// We still respect max buffer limits and may fall back to passthrough for large bodies.
-		shouldBuffer = func(http.Header) bool { return true }
+		mrw := httputils.NewModifyResponseWriter(w, r, exec.modifyResponse)
+		next(mrw, r)
+		return
 	}
-	lrm := httputils.NewLazyResponseModifier(w, shouldBuffer)
+
+	lrm := httputils.NewLazyResponseModifier(w, canBufferAndModifyResponseBody)
 	lrm.SetMaxBufferedBytes(maxModifiableBody)
 	defer func() {
 		_, err := lrm.FlushRelease()