Add CallToolResult.ToChatMessage extension method (#575)

To simplify non-AIFunction usage getting a ChatMessage with the appropriate FunctionResultContent.

tests/ModelContextProtocol.Tests/AIContentExtensionsTests.cs

@@ -0,0 +1,28 @@
+using Microsoft.Extensions.AI;
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+
+namespace ModelContextProtocol.Tests;
+
+public class AIContentExtensionsTests
+{
+    [Fact]
+    public void CallToolResult_ToChatMessage_ProducesExpectedAIContent()
+    {
+        CallToolResult toolResult = new() { Content = [new TextContentBlock { Text = "This is a test message." }] };
+
+        Assert.Throws<ArgumentNullException>(() => AIContentExtensions.ToChatMessage(null!, "call123"));
+        Assert.Throws<ArgumentNullException>(() => AIContentExtensions.ToChatMessage(toolResult, null!));
+
+        ChatMessage message = AIContentExtensions.ToChatMessage(toolResult, "call123");
+        
+        Assert.NotNull(message);
+        Assert.Equal(ChatRole.Tool, message.Role);
+        
+        FunctionResultContent frc = Assert.IsType<FunctionResultContent>(Assert.Single(message.Contents));
+        Assert.Same(toolResult, frc.RawRepresentation);
+        Assert.Equal("call123", frc.CallId);
+        JsonElement result = Assert.IsType<JsonElement>(frc.Result);
+        Assert.Contains("This is a test message.", result.ToString());
+    }
+}