refactor(gateway): remove parsed request object graphs

Keep large gateway payloads as raw body ranges and bind OpenAI parsed-body caches to the body bytes so failover and mapping do not reuse stale mutable state.

backend/internal/handler/openai_gateway_handler.go

@@ -954,7 +954,7 @@ func (h *OpenAIGatewayHandler) validateFunctionCallOutputRequest(c *gin.Context,
 		return true
 	}
 
-	c.Set(service.OpenAIParsedRequestBodyKey, reqBody)
+	service.CacheOpenAIParsedRequestBody(c, body, reqBody)
 	validation := service.ValidateFunctionCallOutputContext(reqBody)
 	if !validation.HasFunctionCallOutput {
 		return true