.NET: Add samples to show how to create an AIAgent for each provider. (#525)

* Add samples to show how to create an AIAgent for each provider.

* Update dotnet/samples/HowToCreateAnAIAgentByProvider/README.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Address PR comments

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

dotnet/samples/HowToCreateAnAIAgentByProvider/AIAgent_With_ONNX/Program.cs

@@ -0,0 +1,20 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+// This sample shows how to create and use a simple AI agent with ONNX as the backend.
+
+using System;
+using Microsoft.Extensions.AI.Agents;
+using Microsoft.ML.OnnxRuntimeGenAI;
+
+// E.g. C:\repos\Phi-4-mini-instruct-onnx\cpu_and_mobile\cpu-int4-rtn-block-32-acc-level-4
+var modelPath = Environment.GetEnvironmentVariable("ONNX_MODEL_PATH") ?? throw new InvalidOperationException("ONNX_MODEL_PATH is not set.");
+
+const string JokerName = "Joker";
+const string JokerInstructions = "You are good at telling jokes.";
+
+// Get a chat client for ONNX and use it to construct an AIAgent.
+using OnnxRuntimeGenAIChatClient chatClient = new(modelPath);
+AIAgent agent = new ChatClientAgent(chatClient, JokerInstructions, JokerName);
+
+// Invoke the agent and output the text result.
+Console.WriteLine(await agent.RunAsync("Tell me a joke about a pirate."));