.NET: use valid Gemini model (#2695)

* use proper Gemini model

* use named parameters

---------

Co-authored-by: Roger Barreto <[email protected]>

Author: jochenkirstaetter

dotnet/samples/GettingStarted/AgentProviders/Agent_With_GoogleGemini/Program.cs

@@ -11,7 +11,7 @@
 const string JokerName = "JokerAgent";
 
 string apiKey = Environment.GetEnvironmentVariable("GOOGLE_GENAI_API_KEY") ?? throw new InvalidOperationException("Please set the GOOGLE_GENAI_API_KEY environment variable.");
-string model = Environment.GetEnvironmentVariable("GOOGLE_GENAI_MODEL") ?? "gemini-2.5-fast";
+string model = Environment.GetEnvironmentVariable("GOOGLE_GENAI_MODEL") ?? "gemini-2.5-flash";
 
 // Using a Google GenAI IChatClient implementation
 // Until the PR https://github.com/googleapis/dotnet-genai/pull/81 is not merged this option
@@ -28,7 +28,7 @@
 // Using a community driven Mscc.GenerativeAI.Microsoft package
 
 ChatClientAgent agentCommunity = new(
-    new GeminiChatClient(apiKey, model),
+    new GeminiChatClient(apiKey: apiKey, model: model),
     name: JokerName,
     instructions: JokerInstructions);