Updated dotnet client sample

Without this package, the `AsBuilder()` method cannot be found.

Author: AustinWise

docs/quickstart/client.mdx

@@ -1399,6 +1399,7 @@ Then, add the required dependencies to your project:
 dotnet add package ModelContextProtocol --prerelease
 dotnet add package Anthropic.SDK
 dotnet add package Microsoft.Extensions.Hosting
+dotnet add package Microsoft.Extensions.AI
 ```
 
 ## Setting up your API key