improve: enhance GitLab provider error handling and attribute access safety

- Add try-catch block around GitLab instance creation for better error handling
- Use getattr() for safer attribute access in _prepare_clone_url_with_token method
- Improve authentication failure debugging with clearer error messages

pr_agent/git_providers/gitlab_provider.py

@@ -39,6 +39,7 @@ class GitLabProvider(GitProvider):
         auth_method = self._get_auth_method(gitlab_url)
         
         # Create GitLab instance based on authentication method
+        try:
             if auth_method == "oauth_token":
                 self.gl = gitlab.Gitlab(
                     url=gitlab_url,
@@ -49,6 +50,9 @@ class GitLabProvider(GitProvider):
                     url=gitlab_url,
                     private_token=gitlab_access_token
                 )
+        except Exception as e:
+            get_logger().error(f"Failed to create GitLab instance: {e}")
+            raise ValueError(f"Unable to authenticate with GitLab: {e}")
         self.max_comment_chars = 65000
         self.id_project = None
         self.id_mr = None
@@ -719,7 +723,7 @@ class GitLabProvider(GitProvider):
             get_logger().error(f"Repo URL: {repo_url_to_clone} is not a valid gitlab URL.")
             return None
         (scheme, base_url) = repo_url_to_clone.split("gitlab.")
-        access_token = self.gl.oauth_token or self.gl.private_token
+        access_token = getattr(self.gl, 'oauth_token', None) or getattr(self.gl, 'private_token', None)
         if not all([scheme, access_token, base_url]):
             get_logger().error(f"Either no access token found, or repo URL: {repo_url_to_clone} "
                                f"is missing prefix: {scheme} and/or base URL: {base_url}.")