[java][native] Add tests for oneOf form parameters (#16487)

* add tests for oneOf form parameters

* update samples

samples/client/echo_api/java/native/api/openapi.yaml

@@ -62,6 +62,27 @@ paths:
       - form
       x-content-type: application/x-www-form-urlencoded
       x-accepts: text/plain
+  /form/oneof:
+    post:
+      description: Test form parameter(s) for oneOf schema
+      operationId: test/form/oneof
+      requestBody:
+        content:
+          application/x-www-form-urlencoded:
+            schema:
+              $ref: '#/components/schemas/test_form_oneof_request'
+      responses:
+        "200":
+          content:
+            text/plain:
+              schema:
+                type: string
+          description: Successful operation
+      summary: Test form parameter(s) for oneOf schema
+      tags:
+      - form
+      x-content-type: application/x-www-form-urlencoded
+      x-accepts: text/plain
   /header/integer/boolean/string:
     get:
       description: Test header parameter(s)
@@ -667,6 +688,26 @@ components:
         string_form:
           type: string
       type: object
+    test_form_oneof_request_oneOf:
+      properties:
+        form1:
+          type: string
+        form2:
+          type: integer
+      type: object
+    test_form_oneof_request_oneOf_1:
+      properties:
+        form3:
+          type: string
+        form4:
+          type: boolean
+      type: object
+    test_form_oneof_request:
+      oneOf:
+      - $ref: '#/components/schemas/test_form_oneof_request_oneOf'
+      - $ref: '#/components/schemas/test_form_oneof_request_oneOf_1'
+      - $ref: '#/components/schemas/Tag'
+      type: object
     test_query_style_form_explode_true_array_string_query_object_parameter:
       properties:
         values: