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

When a schema sets `x-keep-typed-in-additional-properties: true`, each typed
property setter also calls putAdditionalProperty so all fields are accessible
via getAdditionalProperties() alongside untyped fields.

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

Author: charlie-zhang109

.generator/src/generator/templates/modelSimple.j2

@@ -258,6 +258,9 @@ public class {{ name }} {%- if model.get("x-generate-alias-as-model") %} extends
     {%- else %}
     this.{{ variableName }} = {{ variableName }};
     {%- endif %}
+    {%- if model.get("x-keep-typed-in-additional-properties") %}
+    putAdditionalProperty(JSON_PROPERTY_{{ attr|snake_case|upper }}, {%- if not isRequired and isNullable %} this.{{ variableName }}.orElse(null){%- else %} {{ variableName }}{%- endif %});
+    {%- endif %}
   }
   {%- endif %}
   {%- endfor %}