[cpp-pistache] removed model namespace when unused for operations (#775)

* Remove using model namespace when model is unused
* Add comments to clarify introduction of hasModelImport at API/operations level instead of operation/vendorExtensions level.

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/CppPistacheServerCodegen.java

@@ -246,11 +246,13 @@ public Map<String, Object> postProcessOperationsWithModels(Map<String, Object> o
             }
             op.vendorExtensions.put("x-codegen-pistache-consumesJson", consumeJson);
             op.vendorExtensions.put("x-codegen-pistache-isParsingSupported", isParsingSupported);
+
+            // Check if any one of the operations needs a model, then at API file level, at least one model has to be included.
             for(String hdr : op.imports) {
                 if(importMapping.containsKey(hdr)) {
                     continue;
                 }
-                additionalProperties.put("hasModelImport", true);
+                operations.put("hasModelImport", true);
             }
         }