This PR fixes the SYSLIB0014 obsolete warning in DownloadedCache.cs by replacing the deprecated WebClient with HttpClient.

Problem:
The code was using the obsolete WebClient.DownloadFile() method which generates this warning:

warning SYSLIB0014: 'WebClient.WebClient()' is obsolete: 'WebRequest, HttpWebRequest, ServicePoint, and WebClient are obsolete. Use HttpClient instead.'

Solution:

• Replaced new System.Net.WebClient().DownloadFile(url, filename) with HttpClient implementation
• Added a static HttpClient instance for performance and proper resource management
• Used .GetAwaiter().GetResult() to maintain the existing synchronous API contract
• Added proper HTTP error handling with response.EnsureSuccessStatusCode()

Key Changes:

// Before:
new System.Net.WebClient().DownloadFile(url, filename);

// After:
using (var response = httpClient.GetAsync(url).GetAwaiter().GetResult()) {
    response.EnsureSuccessStatusCode();
    using (var fileStream = File.Create(filename))
    using (var httpStream = response.Content.ReadAsStreamAsync().GetAwaiter().GetResult()) {
        httpStream.CopyTo(fileStream);
    }
}

The changes maintain full backward compatibility - the GetAsFile() method continues to be synchronous and all existing calling code works unchanged.

Fixes #10406.

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