Add deprecation warning to Microsoft.Extensions.ApiDescription.Client MSBuild package (#62646)

Co-authored-by: captainsafia <1857993+captainsafia@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: Copilot

src/Tools/Extensions.ApiDescription.Client/src/build/Microsoft.Extensions.ApiDescription.Client.props

@@ -3,6 +3,11 @@
   <PropertyGroup>
     <_ApiDescriptionClientAssemblyPath>$(MSBuildThisFileDirectory)../tasks/netstandard2.0/Microsoft.Extensions.ApiDescription.Client.dll</_ApiDescriptionClientAssemblyPath>
   </PropertyGroup>
+  
+  <!-- Emit deprecation warning for this package -->
+  <Target Name="_WarnMicrosoftExtensionsApiDescriptionClientDeprecated" BeforeTargets="BeforeCompile;Build;Restore">
+    <Warning Code="ASPDEPR001" Text="The 'Microsoft.Extensions.ApiDescription.Client' package is deprecated and will be removed in a future release." HelpLink="https://aka.ms/aspnet/deprecate/001" />
+  </Target>
   <UsingTask TaskName="GetCurrentOpenApiReference" AssemblyFile="$(_ApiDescriptionClientAssemblyPath)" />
   <UsingTask TaskName="GetOpenApiReferenceMetadata" AssemblyFile="$(_ApiDescriptionClientAssemblyPath)" />