Support x-keep-typed-in-additional-properties in Ruby model generator

When a schema sets `x-keep-typed-in-additional-properties: true`, also add
typed property keys to `additional_properties` so all fields are accessible
from a single map regardless of when they were added to the spec.

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

Author: charlie-zhang109

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

@@ -82,6 +82,9 @@
           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