fix: filter domain assembly scans to Default ALC to prevent type identity mismatches

Plugin assemblies loaded via PluginHost into isolated AssemblyLoadContexts
produce distinct types even from the same DLL. When AppDomain.GetAssemblies()
returns both Default and plugin-ALC copies, DI registration and IOptions<T>
resolution silently fail (e.g. ValkeyTransportOptions defaulting to localhost).

Applied AssemblyLoadContext.Default filter to all 7 assembly discovery sites:
- MessagingServiceCollectionExtensions (transport plugin scan)
- StellaRouterIntegrationHelper (transport plugin loader)
- Gateway.WebService Program.cs (startup transport scan)
- GeneratedEndpointDiscoveryProvider (endpoint provider scan)
- ReflectionEndpointDiscoveryProvider (endpoint attribute scan)
- ServiceCollectionExtensions (schema provider scan)
- MigrationModulePluginDiscovery (migration plugin scan)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/Scanner/__Tests/StellaOps.Scanner.Cartographer.Tests/CartographerProgramTests.cs

@@ -0,0 +1,46 @@
+using System.Net;
+using FluentAssertions;
+using Microsoft.AspNetCore.Mvc.Testing;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.Options;
+using Xunit;
+
+namespace StellaOps.Cartographer.Tests;
+
+public class CartographerProgramTests
+{
+    [Fact]
+    public async Task HealthEndpoints_ReturnOk()
+    {
+        using var factory = new WebApplicationFactory<StellaOps.Cartographer.CartographerEntryPoint>();
+        using var client = factory.CreateClient();
+        var cancellationToken = TestContext.Current.CancellationToken;
+
+        var health = await client.GetAsync("/healthz", cancellationToken);
+        var ready = await client.GetAsync("/readyz", cancellationToken);
+
+        health.StatusCode.Should().Be(HttpStatusCode.OK);
+        ready.StatusCode.Should().Be(HttpStatusCode.OK);
+    }
+
+    [Fact]
+    public void AuthorityOptions_InvalidIssuer_ThrowsOnStart()
+    {
+        using var factory = new WebApplicationFactory<StellaOps.Cartographer.CartographerEntryPoint>().WithWebHostBuilder(builder =>
+        {
+            builder.ConfigureAppConfiguration((_, config) =>
+            {
+                var settings = new Dictionary<string, string?>
+                {
+                    ["Cartographer:Authority:Enabled"] = "true",
+                    ["Cartographer:Authority:Issuer"] = "invalid"
+                };
+
+                config.AddInMemoryCollection(settings);
+            });
+        });
+
+        Action act = () => factory.CreateClient();
+        act.Should().Throw<OptionsValidationException>();
+    }
+}