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/Platform/__Libraries/StellaOps.Platform.Database/MigrationModulePluginDiscovery.cs

@@ -72,12 +72,12 @@ internal static class MigrationModulePluginDiscovery
 
     private static IEnumerable<IMigrationModulePlugin> DiscoverPlugins()
     {
-        foreach (var assembly in AppDomain.CurrentDomain.GetAssemblies().OrderBy(static a => a.FullName, StringComparer.Ordinal))
+        // Filter to Default ALC to avoid type identity mismatches from isolated plugin ALCs.
+        foreach (var assembly in AppDomain.CurrentDomain.GetAssemblies()
+                     .Where(static a => !a.IsDynamic
+                                        && AssemblyLoadContext.GetLoadContext(a) == AssemblyLoadContext.Default)
+                     .OrderBy(static a => a.FullName, StringComparer.Ordinal))
         {
-            if (assembly.IsDynamic)
-            {
-                continue;
-            }
 
             foreach (var type in GetLoadableTypes(assembly))
             {