Add CopilotSkipCliDownload option to .NET SDK

Add MSBuild property CopilotSkipCliDownload that when set to true skips
automatic download and bundling of the Copilot CLI binary. This is useful
for users who install the CLI separately (e.g., via npm or a package manager)
and want to avoid the build-time download.

When skipped, users must ensure COPILOT_CLI_PATH or the CliPath option
points to a valid CLI installation at runtime.

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

Author: SteveSandersonMS

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

@@ -55,8 +55,21 @@
     <CopilotCliDownloadTimeout Condition="'$(CopilotCliDownloadTimeout)' == ''">600</CopilotCliDownloadTimeout>
   </PropertyGroup>
 
+  <!-- Skip automatic download and bundling of the Copilot CLI binary.
+       Set CopilotSkipCliDownload to true in your .csproj or Directory.Build.props if you install
+       the Copilot CLI separately (e.g., via npm or a package manager) and don't want the SDK
+       to download and bundle it.
+       
+       When skipped, you must provide a valid CLI path at runtime via the CliPath option in
+       CopilotClientOptions. If CliPath is not set and the bundled CLI is not found, the SDK
+       will throw an InvalidOperationException when starting the client.
+       
+       <PropertyGroup>
+         <CopilotSkipCliDownload>true</CopilotSkipCliDownload>
+       </PropertyGroup> -->
+
   <!-- Download and extract CLI binary -->
-  <Target Name="_DownloadCopilotCli" BeforeTargets="BeforeBuild" Condition="'$(_CopilotPlatform)' != ''">
+  <Target Name="_DownloadCopilotCli" BeforeTargets="BeforeBuild" Condition="'$(CopilotSkipCliDownload)' != 'true' And '$(_CopilotPlatform)' != ''">
     <Error Condition="'$(CopilotCliVersion)' == ''" Text="CopilotCliVersion is not set. The GitHub.Copilot.SDK.props file may be missing from the NuGet package." />
 
     <!-- Compute paths using version (now available) -->
@@ -90,7 +103,7 @@
   </Target>
 
   <!-- Copy CLI binary to output runtimes folder and register for transitive copy -->
-  <Target Name="_CopyCopilotCliToOutput" AfterTargets="Build" DependsOnTargets="_DownloadCopilotCli" Condition="'$(_CopilotPlatform)' != ''">
+  <Target Name="_CopyCopilotCliToOutput" AfterTargets="Build" DependsOnTargets="_DownloadCopilotCli" Condition="'$(CopilotSkipCliDownload)' != 'true' And '$(_CopilotPlatform)' != ''">
     <PropertyGroup>
       <_CopilotCacheDir>$(IntermediateOutputPath)copilot-cli\$(CopilotCliVersion)\$(_CopilotPlatform)</_CopilotCacheDir>
       <_CopilotCliBinaryPath>$(_CopilotCacheDir)\$(_CopilotBinary)</_CopilotCliBinaryPath>
@@ -101,7 +114,7 @@
   </Target>
 
   <!-- Register CLI binary as content so it flows through project references -->
-  <Target Name="_RegisterCopilotCliForCopy" BeforeTargets="GetCopyToOutputDirectoryItems" DependsOnTargets="_DownloadCopilotCli" Condition="'$(_CopilotPlatform)' != ''">
+  <Target Name="_RegisterCopilotCliForCopy" BeforeTargets="GetCopyToOutputDirectoryItems" DependsOnTargets="_DownloadCopilotCli" Condition="'$(CopilotSkipCliDownload)' != 'true' And '$(_CopilotPlatform)' != ''">
     <PropertyGroup>
       <_CopilotCacheDir>$(IntermediateOutputPath)copilot-cli\$(CopilotCliVersion)\$(_CopilotPlatform)</_CopilotCacheDir>
       <_CopilotCliBinaryPath>$(_CopilotCacheDir)\$(_CopilotBinary)</_CopilotCliBinaryPath>