.NET: Add DevUI package for .NET (#1603)

* Implement DevUI

* Review feedback

* Fix build

dotnet/samples/GettingStarted/DevUI/DevUI_Step01_BasicUsage/README.md

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+# DevUI Step 01 - Basic Usage
+
+This sample demonstrates how to add the DevUI to an ASP.NET Core application with AI agents.
+
+## What is DevUI?
+
+The DevUI provides an interactive web interface for testing and debugging AI agents during development.
+
+## Configuration
+
+Set the following environment variables:
+
+- `AZURE_OPENAI_ENDPOINT` - Your Azure OpenAI endpoint URL (required)
+- `AZURE_OPENAI_DEPLOYMENT_NAME` - Your deployment name (defaults to "gpt-4o-mini")
+
+## Running the Sample
+
+1. Set your Azure OpenAI credentials as environment variables
+2. Run the application:
+   ```bash
+   dotnet run
+   ```
+3. Open your browser to https://localhost:50516/devui
+4. Select an agent or workflow from the dropdown and start chatting!
+
+## Sample Agents and Workflows
+
+This sample includes:
+
+**Agents:**
+- **assistant** - A helpful assistant
+- **poet** - A creative poet
+- **coder** - An expert programmer
+
+**Workflows:**
+- **review-workflow** - A sequential workflow that generates a response and then reviews it
+
+## Adding DevUI to Your Own Project
+
+To add DevUI to your ASP.NET Core application:
+
+1. Add the DevUI package and hosting packages:
+   ```bash
+   dotnet add package Microsoft.Agents.AI.DevUI
+   dotnet add package Microsoft.Agents.AI.Hosting
+   dotnet add package Microsoft.Agents.AI.Hosting.OpenAI
+   ```
+
+2. Register your agents and workflows:
+   ```csharp
+   var builder = WebApplication.CreateBuilder(args);
+   
+   // Set up your chat client
+   builder.Services.AddChatClient(chatClient);
+   
+   // Register agents
+   builder.AddAIAgent("assistant", "You are a helpful assistant.");
+   
+   // Register workflows
+   var agent1Builder = builder.AddAIAgent("workflow-agent1", "You are agent 1.");
+   var agent2Builder = builder.AddAIAgent("workflow-agent2", "You are agent 2.");
+   builder.AddSequentialWorkflow("my-workflow", [agent1Builder, agent2Builder])
+       .AddAsAIAgent();
+   ```
+
+3. Add DevUI services and map the endpoint:
+   ```csharp
+   builder.AddDevUI();
+   var app = builder.Build();
+   
+   app.MapDevUI();
+   
+   // Add required endpoints
+   app.MapEntities();
+   app.MapOpenAIResponses();
+   app.MapOpenAIConversations();
+   
+   app.Run();
+   ```
+
+4. Navigate to `/devui` in your browser