Fix C# samples CI: restore local-sdk NuGet source

The 'local-sdk' source (../../local-packages) was removed in PR #573 and
the ORT-Nightly feed was removed in PR #633, leaving only nuget.org.
Since Microsoft.AI.Foundry.Local is not published to nuget.org, all C#
sample builds fail during NuGet restore.

Restore the local-sdk source with package source mapping so the CI
'Build SDK from source' step output is discoverable by sample builds.

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

Author: ruiren_microsoft

samples/cs/nuget.config

@@ -3,5 +3,15 @@
   <packageSources>
     <clear />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
+    <add key="local-sdk" value="../../local-packages" />
   </packageSources>
+  <packageSourceMapping>
+    <packageSource key="nuget.org">
+      <package pattern="*" />
+    </packageSource>
+    <packageSource key="local-sdk">
+      <package pattern="Microsoft.AI.Foundry.Local" />
+      <package pattern="Microsoft.AI.Foundry.Local.WinML" />
+    </packageSource>
+  </packageSourceMapping>
 </configuration>