fix(messaging): give a clear startup error when no transport is configured

Author: Vulthil

src/Vulthil.Messaging/ConsumerHostedService.cs

@@ -6,9 +6,12 @@ internal sealed class ConsumerHostedService : BackgroundService
 {
     private readonly ITransport _transport;
 
-    public ConsumerHostedService(ITransport transport)
+    public ConsumerHostedService(IEnumerable<ITransport> transports)
     {
-        _transport = transport;
+        _transport = transports.LastOrDefault()
+            ?? throw new InvalidOperationException(
+                "No messaging transport is registered. Call a transport extension such as .UseRabbitMq(...) " +
+                "inside AddMessaging(...) (or .UseTestHarness() in a test).");
     }
 
     protected override Task ExecuteAsync(CancellationToken stoppingToken) => _transport.StartAsync(stoppingToken);

src/Vulthil.Messaging/DependencyInjection.cs

@@ -22,6 +22,9 @@ public static class DependencyInjection
     /// Code registrations performed inside the configurator action merge onto the loaded values and take precedence.
     /// Built-in consume filters (see <see cref="ConsumeFilterOptions"/>) are registered before the configurator
     /// action runs, so user-registered filters compose INSIDE the defaults.
+    /// <para>A transport must be configured — e.g. <c>UseRabbitMq(...)</c> inside the configurator, or registered
+    /// separately (as <c>ReplaceTransportWithTestHarness</c> does) — otherwise the host fails to start with a clear
+    /// error.</para>
     /// </remarks>
     /// <param name="builder">The host application builder.</param>
     /// <param name="messagingConfiguratorAction">An action to configure messaging through <see cref="IMessagingConfigurator"/>.</param>

tests/Vulthil.Messaging.Tests/MessagingConfiguratiorTests.cs

@@ -29,6 +29,18 @@ public void AddMessagingShouldRegisterConsumerHostedService()
         hostedServices[0].Lifetime.ShouldBe(ServiceLifetime.Singleton);
     }
 
+    [Fact]
+    public async Task StartingTheHostWithoutATransportThrowsAClearError()
+    {
+        // Arrange — messaging configured, but no transport (no UseRabbitMq/UseTestHarness).
+        Target.AddMessaging(x => x.ConfigureQueue("orders", _ => { }));
+        using var host = Target.Build();
+
+        // Act & Assert
+        var exception = await Should.ThrowAsync<InvalidOperationException>(() => host.StartAsync(CancellationToken));
+        exception.Message.ShouldContain("No messaging transport is registered");
+    }
+
     [Fact]
     public void AddMessagingShouldRegisterMessageConfigurationProvider()
     {