Better handling of allOf in request body (#16991)

* better handling of allOf in request body, add tests

* additional checks

* fix description

samples/client/echo_api/csharp-restsharp/api/openapi.yaml

@@ -403,6 +403,22 @@ paths:
       summary: Test body parameter(s)
       tags:
       - body
+  /echo/body/allOf/Pet:
+    post:
+      description: Test body parameter(s)
+      operationId: test/echo/body/allOf/Pet
+      requestBody:
+        $ref: '#/components/requestBodies/AllOfPet'
+      responses:
+        "200":
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Pet'
+          description: Successful operation
+      summary: Test body parameter(s)
+      tags:
+      - body
   /echo/body/Pet/response_string:
     post:
       description: Test empty response body
@@ -513,6 +529,13 @@ components:
           schema:
             $ref: '#/components/schemas/Pet'
       description: Pet object that needs to be added to the store
+    AllOfPet:
+      content:
+        application/json:
+          schema:
+            allOf:
+            - $ref: '#/components/schemas/Pet'
+      description: Pet object that needs to be added to the store
     Tag:
       content:
         application/json: