fix the multipart get part ETag

weed/s3api/s3api_object_handlers.go

@@ -1715,11 +1715,13 @@ func (s3a *S3ApiServer) setResponseHeaders(w http.ResponseWriter, entry *filer_p
 	w.Header().Set("Content-Length", strconv.FormatInt(totalSize, 10))
 	w.Header().Set("Accept-Ranges", "bytes")
 
-	// Set ETag
+	// Set ETag (but don't overwrite if already set, e.g., for part-specific GET requests)
+	if w.Header().Get("ETag") == "" {
 		etag := filer.ETag(entry)
 		if etag != "" {
 			w.Header().Set("ETag", "\""+etag+"\"")
 		}
+	}
 
 	// Set Last-Modified in RFC1123 format
 	if entry.Attributes != nil {