S3: quote ETag in CopyObject and UploadPartCopy XML responses (#11624)

* s3api: extract quoteETag from setEtag

Consolidate ETag quoting so the XML response builders can share it.

* s3api: quote ETag in CopyObjectResult XML

AWS returns the ETag quoted in the copy result body, matching the ETag header. Fixes seaweedfs/seaweedfs#11622

* s3api: quote ETag in CopyPartResult XML

UploadPartCopy returned the raw ETag in the XML body while the response header and other APIs return it quoted. Fixes seaweedfs/seaweedfs#11622

* s3api: test ETag quoting in copy responses

* s3api: assert quoted ETag on the wire in copy response tests

* s3api: use strconv.Quote in quoteETag

Addresses CodeQL 'potentially unsafe quoting' on string concatenation.

weed/s3api/s3api_object_handlers_put.go

@@ -1397,14 +1397,17 @@ func (s3a *S3ApiServer) resolveFileMode(r *http.Request) uint32 {
 
 func setEtag(w http.ResponseWriter, etag string) {
 	if etag != "" {
-		if strings.HasPrefix(etag, "\"") {
-			w.Header()["ETag"] = []string{etag}
-		} else {
-			w.Header()["ETag"] = []string{"\"" + etag + "\""}
-		}
+		w.Header()["ETag"] = []string{quoteETag(etag)}
 	}
 }
 
+func quoteETag(etag string) string {
+	if etag == "" || strings.HasPrefix(etag, "\"") {
+		return etag
+	}
+	return strconv.Quote(etag)
+}
+
 // setSSEResponseHeaders sets appropriate SSE response headers based on encryption type
 func (s3a *S3ApiServer) setSSEResponseHeaders(w http.ResponseWriter, r *http.Request, sseMetadata SSEResponseMetadata) {
 	switch sseMetadata.SSEType {