NuGet: drop Liara from root nuget.config too (Nexus-only everywhere)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

nuget.config

@@ -2,9 +2,9 @@
 <configuration>
   <packageSources>
     <clear />
-    <!-- Soroush Nexus mirror (primary) + Liara mirror (fallback) — nuget.org is filtered. -->
+    <!-- Single source: Soroush Nexus mirror. No Liara fallback — NuGet probes every
+         listed source's index, so a dead fallback (500) aborts the whole restore. -->
     <add key="nexus" value="https://mirror.soroushasadi.com/repository/nuget-group/index.json" protocolVersion="3" />
-    <add key="liara" value="https://package-mirror.liara.ir/repository/nuget/index.json" protocolVersion="3" />
   </packageSources>
   <config>
     <add key="http_retry_count" value="6" />