feat(generator): expose NinoBuiltInTypesRegistration as public

Changed NinoBuiltInTypesRegistration class from internal to public to allow
other assemblies to manually invoke built-in type registration initialization.

This enables external assemblies to explicitly control when built-in type
registrations are initialized, useful for scenarios where automatic initialization
via ModuleInitializer or static constructors may not be desirable.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: JasonXuDeveloper

src/Nino.Generator/BuiltInType/NinoBuiltInTypesGenerator.cs

@@ -176,7 +176,7 @@ public static partial class Deserializer
 
                      namespace {{curNamespace}}
                      {
-                         internal static class NinoBuiltInTypesRegistration
+                         public static class NinoBuiltInTypesRegistration
                          {
                              private static bool _initialized;
                              private static object _lock = new object();