[C++][Pistache] Compile error when nesting references (#16711) (#18586)

When components/schema/<object> reference other objects, validate() was
getting called on the referenced objects with no arguments. The return
value was void, but checked as if it was a boolean value.

samples/server/petstore/cpp-pistache-nested-schema-refs/api/StoreApi.h

@@ -0,0 +1,77 @@
+/**
+* Test swagger file
+* No description provided (generated by Openapi Generator https://github.com/openapitools/openapi-generator)
+*
+* The version of the OpenAPI document: 1.0.0
+* 
+*
+* NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+* https://openapi-generator.tech
+* Do not edit the class manually.
+*/
+/*
+ * StoreApi.h
+ *
+ * 
+ */
+
+#ifndef StoreApi_H_
+#define StoreApi_H_
+
+
+#include "ApiBase.h"
+
+#include <pistache/http.h>
+#include <pistache/router.h>
+#include <pistache/http_headers.h>
+
+#include <optional>
+#include <utility>
+
+#include "OuterType.h"
+
+namespace org::openapitools::server::api
+{
+
+class  StoreApi : public ApiBase {
+public:
+    explicit StoreApi(const std::shared_ptr<Pistache::Rest::Router>& rtr);
+    ~StoreApi() override = default;
+    void init() override;
+
+    static const std::string base;
+
+private:
+    void setupRoutes();
+
+    void get_nested_object_handler(const Pistache::Rest::Request &request, Pistache::Http::ResponseWriter response);
+    void store_api_default_handler(const Pistache::Rest::Request &request, Pistache::Http::ResponseWriter response);
+
+    /// <summary>
+    /// Helper function to handle unexpected Exceptions during Parameter parsing and validation.
+    /// May be overridden to return custom error formats. This is called inside a catch block.
+    /// Important: When overriding, do not call `throw ex;`, but instead use `throw;`.
+    /// </summary>
+    virtual std::pair<Pistache::Http::Code, std::string> handleParsingException(const std::exception& ex) const noexcept;
+
+    /// <summary>
+    /// Helper function to handle unexpected Exceptions during processing of the request in handler functions.
+    /// May be overridden to return custom error formats. This is called inside a catch block.
+    /// Important: When overriding, do not call `throw ex;`, but instead use `throw;`.
+    /// </summary>
+    virtual std::pair<Pistache::Http::Code, std::string> handleOperationException(const std::exception& ex) const noexcept;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <remarks>
+    /// 
+    /// </remarks>
+    virtual void get_nested_object(Pistache::Http::ResponseWriter &response) = 0;
+
+};
+
+} // namespace org::openapitools::server::api
+
+#endif /* StoreApi_H_ */
+