Differentiate user-agent headers for library vs dnup HTTP calls

[Copilot]

dotnet up makes HTTP requests from two sources: the Microsoft.Deployment.DotNet.Releases library (for release metadata) and DotnetArchiveDownloader (for archive downloads). Both were using the same or no custom user-agent, making usage metrics indistinguishable.

Changes

• New ReleaseLibraryInitializer: Uses reflection to set custom user-agent on the library's internal static HttpClient

  • Library calls: dotnetup-library/{version}
  • Direct dnup calls: dotnetup-dotnet-installer/{version} (unchanged)

• Initialization: Called early in InstallerOrchestratorSingleton.Install() and SdkInstallCommand.Execute() before first library use

• Error handling: Graceful degradation if reflection fails; logs to Debug output but doesn't break functionality

Implementation

The library's Utils.s_httpClient is accessed via reflection since it's internal:

var utilsType = Type.GetType("Microsoft.Deployment.DotNet.Releases.Utils, Microsoft.Deployment.DotNet.Releases");
var httpClientField = utilsType.GetField("s_httpClient", BindingFlags.Static | BindingFlags.NonPublic);
var httpClient = httpClientField.GetValue(null) as HttpClient;
httpClient.DefaultRequestHeaders.UserAgent.ParseAdd($"dotnetup-library/{version}");

Thread-safe singleton pattern with _initialized flag prevents redundant reflection attempts.

Original prompt

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This section details on the original issue you should resolve

<issue_title>dotnet up should use a separate header for library calls</issue_title>
<issue_description>dotnet up uses a custom agent header for http requests so our websites can figure out usage metrics and also prevent our client from being throttled. It uses the same metadata regardless of if the request if coming from the library or dnup itself in the ReleaseManifest, so we should try to differentiate the two. </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes dotnet up should use a separate header for library calls #51586

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