use model class for request body with additionalProperties (#16613)

* try out simple fix

* add test case

* generate samples

samples/client/petstore/java/apache-httpclient/api/openapi.yaml

@@ -1031,6 +1031,25 @@ paths:
       - fake
       x-content-type: application/json
       x-accepts: application/json
+  /fake/inline-freeform-additionalProperties:
+    post:
+      description: ""
+      operationId: testInlineFreeformAdditionalProperties
+      requestBody:
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/testInlineFreeformAdditionalProperties_request'
+        description: request body
+        required: true
+      responses:
+        "200":
+          description: successful operation
+      summary: test inline free-form additionalProperties
+      tags:
+      - fake
+      x-content-type: application/json
+      x-accepts: application/json
   /fake/nullable:
     post:
       description: ""
@@ -2191,6 +2210,12 @@ components:
       - param
       - param2
       type: object
+    testInlineFreeformAdditionalProperties_request:
+      additionalProperties: true
+      properties:
+        someProperty:
+          type: string
+      type: object
     uploadFileWithRequiredFile_request:
       properties:
         additionalMetadata: