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/Router/StellaOps.Gateway.WebService/Program.cs

@@ -24,6 +24,7 @@ using StellaOps.Router.Gateway.Middleware;
 using StellaOps.Router.Gateway.OpenApi;
 using StellaOps.Router.Gateway.RateLimit;
 using StellaOps.Router.Gateway.Routing;
+using System.Runtime.Loader;
 
 var builder = WebApplication.CreateBuilder(args);
 
@@ -77,7 +78,8 @@ var transportPluginLoader = new RouterTransportPluginLoader(
 
 foreach (var assembly in AppDomain.CurrentDomain.GetAssemblies()
              .Where(assembly =>
-                 assembly.GetName().Name?.StartsWith("StellaOps.Router.Transport.", StringComparison.OrdinalIgnoreCase) == true))
+                 assembly.GetName().Name?.StartsWith("StellaOps.Router.Transport.", StringComparison.OrdinalIgnoreCase) == true
+                 && AssemblyLoadContext.GetLoadContext(assembly) == AssemblyLoadContext.Default))
 {
     transportPluginLoader.LoadFromAssembly(assembly);
 }