Update googleapis_auth/lib/src/sts_auth_client.dart

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: kevmoo

googleapis_auth/lib/src/sts_auth_client.dart

@@ -151,7 +151,7 @@ class StsAuthClient extends AutoRefreshDelegatingClient {
             token = subjectToken;
           } else {
             throw ServerRequestFailedException(
-              'Subject token field "$fieldName" not found in JSON response.',
+              'Subject token field "$fieldName" not found or not a string in JSON response.',
               responseContent: token,
               statusCode: response.statusCode,
             );