Upgrade Anthropic SDK 12.13.0 -> 12.20.0 to fix M.E.AI incompatibility

Fixes MissingMethodException on WebSearchToolResultContent.get_Results()
caused by Anthropic 12.13.0 being compiled against an older
Microsoft.Extensions.AI.Abstractions version.

Suppress RT0003 in AI.Abstractions.csproj as the transitive reference
from the upgraded Anthropic SDK conflicts with the explicit one.

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

dotnet/Directory.Packages.props

@@ -11,7 +11,7 @@
   </PropertyGroup>
   <ItemGroup>
     <!-- Aspire.* -->
-    <PackageVersion Include="Anthropic" Version="12.13.0" />
+    <PackageVersion Include="Anthropic" Version="12.20.0" />
     <PackageVersion Include="Anthropic.Foundry" Version="0.5.0" />
     <PackageVersion Include="Aspire.Hosting" Version="$(AspireAppHostSdkVersion)" />
     <PackageVersion Include="Aspire.Azure.AI.OpenAI" Version="13.0.0-preview.1.25560.3" />

dotnet/src/Microsoft.Agents.AI.Abstractions/Microsoft.Agents.AI.Abstractions.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <RootNamespace>Microsoft.Agents.AI</RootNamespace>
-    <NoWarn>$(NoWarn);MEAI001</NoWarn>
+    <NoWarn>$(NoWarn);MEAI001;RT0003</NoWarn>
     <IsReleased>true</IsReleased>
   </PropertyGroup>