.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

---------

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

dotnet/samples/HowToCreateAnAIAgentByProvider/AIAgent_With_OpenAIResponses/AIAgent_With_OpenAIResponses.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net9.0</TargetFramework>
+    <LangVersion>12</LangVersion>
+
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>disable</ImplicitUsings>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\..\src\Microsoft.Extensions.AI.Agents.OpenAI\Microsoft.Extensions.AI.Agents.OpenAI.csproj" />
+    <ProjectReference Include="..\..\..\src\Microsoft.Extensions.AI.Agents\Microsoft.Extensions.AI.Agents.csproj" />
+  </ItemGroup>
+
+</Project>

dotnet/samples/HowToCreateAnAIAgentByProvider/AIAgent_With_OpenAIResponses/Program.cs

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+// Copyright (c) Microsoft. All rights reserved.
+
+// This sample shows how to create and use a simple AI agent with OpenAI Responses as the backend.
+
+#pragma warning disable OPENAI001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
+
+using System;
+using Microsoft.Extensions.AI.Agents;
+using OpenAI;
+
+var apiKey = Environment.GetEnvironmentVariable("OPENAI_APIKEY") ?? throw new InvalidOperationException("OPENAI_APIKEY is not set.");
+var modelName = Environment.GetEnvironmentVariable("OPENAI_MODEL") ?? "gpt-4o-mini";
+
+const string JokerName = "Joker";
+const string JokerInstructions = "You are good at telling jokes.";
+
+AIAgent agent = new OpenAIClient(
+    apiKey)
+     .GetOpenAIResponseClient(modelName)
+     .CreateAIAgent(JokerInstructions, JokerName);
+
+// Invoke the agent and output the text result.
+Console.WriteLine(await agent.RunAsync("Tell me a joke about a pirate."));

dotnet/samples/HowToCreateAnAIAgentByProvider/AIAgent_With_OpenAIResponses/README.md

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+# Prerequisites
+
+Before you begin, ensure you have the following prerequisites:
+
+- .NET 8.0 SDK or later
+- OpenAI api key
+
+Set the following environment variables:
+
+```powershell
+$env:OPENAI_APIKEY="*****" # Replace with your OpenAI api key
+$env:OPENAI_MODEL="gpt-4o-mini"  # Optional, defaults to gpt-4o-mini
+```