fix: Auth header when downloading images from tfs (#2877)

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: MrHinsh

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

@@ -132,8 +132,18 @@ private string UploadedAndRetrieveAttachmentLinkUrl(string matchedSourceUri, str
                 {
                     if (!string.IsNullOrEmpty(sourcePersonalAccessToken))
                     {
-                        httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", sourcePersonalAccessToken);
+                        var host = new Uri(matchedSourceUri).Host.ToLowerInvariant();
+                        if (host.Contains("dev.azure.com") || host.Contains("visualstudio.com"))
+                        {
+                            httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", sourcePersonalAccessToken);
+                        }
+                        else
+                        {
+                            string credentials = Convert.ToBase64String(Encoding.ASCII.GetBytes(string.Format("{0}:{1}", "", sourcePersonalAccessToken)));
+                            httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", credentials);
+                        }
                     }
+
                     var result = DownloadFile(httpClient, matchedSourceUri, fullImageFilePath);
                     if (!result.IsSuccessStatusCode)
                     {