Add helper factory methods to Chat

Allows simpler construction of chat message like:

```csharp
using static Devlooped.Extensions.AI.Chat;
// ...
var response = client.GetResponseAsync(
  [Assistant("how can I help?"), User("hello")])
```

Author: kzu

src/Extensions/Chat.cs

@@ -41,6 +41,15 @@ public void Add(string role, string message)
             _ => new ChatRole(role)
         }, message));
 
+    /// <summary>Creates a user message.</summary>
+    public static ChatMessage User(string message) => new(ChatRole.User, message);
+
+    /// <summary>Creates an assistant message.</summary>
+    public static ChatMessage Assistant(string message) => new(ChatRole.Assistant, message);
+
+    /// <summary>Creates a system message.</summary>
+    public static ChatMessage System(string message) => new(ChatRole.System, message);
+
     IEnumerator<ChatMessage> IEnumerable<ChatMessage>.GetEnumerator() => messages.GetEnumerator();
 
     IEnumerator IEnumerable.GetEnumerator() => messages.GetEnumerator();

src/Tests/ChatExtensionsTests.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Devlooped.Extensions.AI;
+using Microsoft.Extensions.AI;
+using static Devlooped.Extensions.AI.Chat;
+
+namespace Devlooped;
+
+public class ChatExtensionsTests
+{
+    [Fact]
+    public void FactoryMethods()
+    {
+        var message = User("hello");
+
+        Assert.Equal(ChatRole.User, message.Role);
+        Assert.Equal("hello", message.Text);
+
+        message = Assistant("hello");
+
+        Assert.Equal(ChatRole.Assistant, message.Role);
+        Assert.Equal("hello", message.Text);
+
+        // Can't use System without qualifying since it's also a namespace.
+        message = Chat.System("hello");
+        Assert.Equal(ChatRole.System, message.Role);
+        Assert.Equal("hello", message.Text);
+    }
+}