fix: copy typed fields into additional_properties after coercion, not before

The previous approach stored the raw pre-coercion hash value. After setters run,
self.attr_name holds the coerced value. We now copy post-coercion so both access
paths return consistent types.

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

Author: charlie-zhang109

.generator/src/generator/templates/model_generic.j2

@@ -82,9 +82,6 @@
           self.additional_properties[k.to_sym] = v
         else
           h[k.to_sym] = v
-{%- if model.get("x-keep-typed-in-additional-properties") %}
-          self.additional_properties[k.to_sym] = v
-{%- endif %}
         end
 {%- else %}
           fail ArgumentError, "`#{k}` is not a valid attribute in `{{ module_name }}::{{ version|upper }}::{{ name }}`. Please check the name to make sure it's valid. List of attributes: " + self.class.attribute_map.keys.inspect
@@ -104,6 +101,11 @@
         {%- endif %}
       end
 {%- endfor %}
+{%- if model.get("x-keep-typed-in-additional-properties") %}
+{%- for attr, definition in model.get("properties", {}).items() %}
+      self.additional_properties[:'{{ attr|attribute_name }}'] = self.{{ attr|attribute_name }} if attributes.key?(:'{{ attr|attribute_name }}')
+{%- endfor %}
+{%- endif %}
     end
 
 {%- set ns = namespace(hasValidation=False) %}