Add User-Agent header to binary download via WebClient

Matches what Node SDK already does on every HTTP request (rails
endpoint POST AND CDN binary GET). Until this commit, the WebClient
binary download was the only request without a UA header, which
made CloudFront/CloudFlare access logs unfilterable by binding+version.

Now every outbound HTTP from this binding sends:

  User-Agent: browserstack-local-csharp/<version>

Same value the HttpClient already sent on the rails endpoint POST.

Follow-up to the code review pass on LOC-6563.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: yashdsaraf

BrowserStackLocal/BrowserStackLocal/BrowserStackTunnel.cs

@@ -242,6 +242,7 @@ public void downloadBinary()
 
       using (var client = new WebClient())
       {
+        client.Headers.Add(HttpRequestHeader.UserAgent, userAgent);
         if (!string.IsNullOrEmpty(proxyHost) && proxyPort > 0)
         {
           client.Proxy = new WebProxy(proxyHost, proxyPort);