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

When a schema sets `x-keep-typed-in-additional-properties: true`, emit a
`_keep_typed_in_additional_properties = True` class attribute. In Python all
fields are already stored in `_data_store` and accessible via `model["key"]`
regardless of type; this marker makes the intent explicit for tooling.

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

Author: charlie-zhang109

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

@@ -74,6 +74,9 @@ class {{ name }}(ModelNormal):
 {%- if model.nullable %}
     _nullable = True
 {%- endif %}
+{%- if model.get("x-keep-typed-in-additional-properties") %}
+    _keep_typed_in_additional_properties = True
+{%- endif %}
 
 {%- if model.properties %}
     @cached_property