tests: add unit tests for CreateMessageRequest, DiscordMessageReference, and DiscordRestClient

refactor: update DiscordRestClient and IDiscordRestClient to use default CancellationToken
refactor: rename MessageReference to DiscordMessageReference and add DiscordMessageReferenceTypes
chore: update WelcomeMessageHandler to use new DiscordMessageReference

src/StevesBot.Worker.Tests/Unit/WelcomeMessageHandlerTests.cs

@@ -0,0 +1,31 @@
+using StevesBot.Worker.Handlers;
+
+namespace StevesBot.Worker.Tests.Unit;
+
+public class WelcomeMessageHandlerTests
+{
+  private readonly Mock<IDiscordRestClient> _mockDiscordRestClient = new();
+  private readonly Mock<ILogger<IDiscordGatewayClient>> _mockLogger = new();
+  private readonly IServiceProvider _serviceProvider;
+
+  public WelcomeMessageHandlerTests()
+  {
+    var serviceCollection = new ServiceCollection();
+    serviceCollection.AddSingleton(_mockDiscordRestClient.Object);
+    serviceCollection.AddSingleton(_mockLogger.Object);
+
+    _serviceProvider = serviceCollection.BuildServiceProvider();
+  }
+
+  [Fact]
+  public async Task HandleAsync_WhenEventIsNotMessageCreateEvent_ItShouldNotCreateMessage()
+  {
+    await WelcomeMessageHandler.HandleAsync(new DiscordEvent(), _serviceProvider);
+
+    _mockDiscordRestClient
+      .Verify(
+        static c => c.CreateMessageAsync(It.IsAny<string>(), It.IsAny<CreateMessageRequest>(), It.IsAny<CancellationToken>()),
+        Times.Never
+      );
+  }
+}