Add CancellationToken propagation to IBuildServerClient async methods

Background

IBuildServerClient defines async methods (GetBuildsAsync, GetBuildArtifactDownloadUrlAsync, ValidateBuildAsync) but none accept a CancellationToken. Long-running GitHub API calls cannot be cancelled when a request is aborted.

Recommended Approach

1. Add CancellationToken cancellationToken = default to all async methods on IBuildServerClient
2. Pass the token through to all HttpClient calls in GitHubActionsBuildServerClient
3. Pass from ASP.NET controller actions via HttpContext.RequestAborted
4. Update AzureDevOpsBuildServerClient to pass the token to its underlying client

Identified in multi-model review of PR #584 and PR #585