fix: Auth header when downloading images from tfs

Updated the logic for setting the `Authorization` header in `HttpClient` to improve security and flexibility. The code now checks if the `sourcePersonalAccessToken` is not empty and if the `matchedSourceUri` contains "tfs/DefaultCollection". If both conditions are met, the token is encoded in Base64 with an empty username; otherwise, the token is set directly if it's not empty.

Author: luxdave

src/MigrationTools.Clients.TfsObjectModel/Tools/TfsEmbededImagesTool.cs

@@ -132,7 +132,15 @@ private string UploadedAndRetrieveAttachmentLinkUrl(string matchedSourceUri, str
                 {
                     if (!string.IsNullOrEmpty(sourcePersonalAccessToken))
                     {
-                        httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", sourcePersonalAccessToken);
+                        if (!string.IsNullOrEmpty(sourcePersonalAccessToken) && matchedSourceUri.Contains("tfs/DefaultCollection"))
+                        {
+                            string credentials = Convert.ToBase64String(Encoding.ASCII.GetBytes(string.Format("{0}:{1}", "", sourcePersonalAccessToken)));
+                            httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", credentials);
+                        }
+                        else if (!string.IsNullOrEmpty(sourcePersonalAccessToken))
+                        {
+                            httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", sourcePersonalAccessToken);
+                        }
                     }
                     var result = DownloadFile(httpClient, matchedSourceUri, fullImageFilePath);
                     if (!result.IsSuccessStatusCode)