Fix .NET SDK CLI download timeout

Increase the default timeout for downloading the Copilot CLI tarball from 100s
(the HttpClient default) to 600s (10 minutes) to handle slow or unreliable
network conditions.

The timeout is configurable via the CopilotCliDownloadTimeout MSBuild property.

Fixes #451

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

dotnet/src/build/GitHub.Copilot.SDK.targets

@@ -48,6 +48,13 @@
     <CopilotNpmRegistryUrl Condition="'$(CopilotNpmRegistryUrl)' == ''">https://registry.npmjs.org</CopilotNpmRegistryUrl>
   </PropertyGroup>
 
+  <!-- Timeout in seconds for downloading the Copilot CLI tarball.
+       Override CopilotCliDownloadTimeout in your .csproj or Directory.Build.props if needed.
+       Default is 600 seconds (10 minutes) to handle slow or unreliable network conditions. -->
+  <PropertyGroup>
+    <CopilotCliDownloadTimeout Condition="'$(CopilotCliDownloadTimeout)' == ''">600</CopilotCliDownloadTimeout>
+  </PropertyGroup>
+
   <!-- Download and extract CLI binary -->
   <Target Name="_DownloadCopilotCli" BeforeTargets="BeforeBuild" Condition="'$(_CopilotPlatform)' != ''">
     <Error Condition="'$(CopilotCliVersion)' == ''" Text="CopilotCliVersion is not set. The GitHub.Copilot.SDK.props file may be missing from the NuGet package." />
@@ -68,6 +75,7 @@
     <MakeDir Directories="$(_CopilotCacheDir)" Condition="!Exists('$(_CopilotCliBinaryPath)')" />
     <Message Importance="high" Text="Downloading Copilot CLI $(CopilotCliVersion) for $(_CopilotPlatform)..." Condition="!Exists('$(_CopilotCliBinaryPath)')" />
     <DownloadFile SourceUrl="$(_CopilotDownloadUrl)" DestinationFolder="$(_CopilotCacheDir)" DestinationFileName="copilot.tgz"
+                  Timeout="$([System.Int32]::Parse('$(CopilotCliDownloadTimeout)'))"
                   Condition="!Exists('$(_CopilotCliBinaryPath)')" />
 
     <!-- Extract using tar (use Windows system tar explicitly to avoid Git bash tar issues) -->