.NET: OpenAI Reasoning Agent sample (#1047)

* OpenAI reasoning agent sample

* Refactor OpenAI agent sample: streamline warning handling and improve output formatting

dotnet/samples/GettingStarted/AgentWithOpenAI/Agent_OpenAI_Step02_Reasoning/Agent_OpenAI_Step02_Reasoning.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net9.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>disable</ImplicitUsings>
+    <NoWarn>$(NoWarn);OPENAI001</NoWarn>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference
+      Include="..\..\..\..\src\Microsoft.Agents.AI.OpenAI\Microsoft.Agents.AI.OpenAI.csproj" />
+  </ItemGroup>
+
+</Project>

dotnet/samples/GettingStarted/AgentWithOpenAI/Agent_OpenAI_Step02_Reasoning/Program.cs

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+// Copyright (c) Microsoft. All rights reserved.
+
+// This sample shows how to create and use an AI agent with reasoning capabilities.
+
+using System;
+using Microsoft.Agents.AI;
+using Microsoft.Extensions.AI;
+using OpenAI;
+using OpenAI.Responses;
+
+var apiKey = Environment.GetEnvironmentVariable("OPENAI_APIKEY") ?? throw new InvalidOperationException("OPENAI_APIKEY is not set.");
+var model = Environment.GetEnvironmentVariable("OPENAI_MODEL") ?? "gpt-5";
+
+var client = new OpenAIClient(apiKey)
+        .GetOpenAIResponseClient(model)
+        .AsIChatClient().AsBuilder()
+        .ConfigureOptions(o =>
+        {
+            o.RawRepresentationFactory = _ => new ResponseCreationOptions()
+            {
+                ReasoningOptions = new()
+                {
+                    ReasoningEffortLevel = ResponseReasoningEffortLevel.Medium,
+                    // Verbosity requires OpenAI verified Organization
+                    ReasoningSummaryVerbosity = ResponseReasoningSummaryVerbosity.Detailed
+                }
+            };
+        }).Build();
+
+AIAgent agent = new ChatClientAgent(client);
+
+Console.WriteLine("1. Non-streaming:");
+var response = await agent.RunAsync("Solve this problem step by step: If a train travels 60 miles per hour and needs to cover 180 miles, how long will the journey take? Show your reasoning.");
+
+Console.WriteLine(response.Text);
+
+Console.WriteLine("Token usage:");
+Console.WriteLine($"Input: {response.Usage?.InputTokenCount}, Output: {response.Usage?.OutputTokenCount}, {string.Join(", ", response.Usage?.AdditionalCounts ?? [])}");
+Console.WriteLine();
+
+Console.WriteLine("2. Streaming");
+await foreach (var update in agent.RunStreamingAsync("Explain the theory of relativity in simple terms."))
+{
+    foreach (var item in update.Contents)
+    {
+        if (item is TextReasoningContent reasoningContent)
+        {
+            Console.Write($"\e[97m{reasoningContent.Text}\e[0m");
+        }
+        else if (item is TextContent textContent)
+        {
+            Console.Write(textContent.Text);
+        }
+    }
+}