[cpp pistache] use model namespace only on Model import (#733)

Don't use model namespaces when not imported

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

@@ -246,6 +246,12 @@ public class CppPistacheServerCodegen extends AbstractCppCodegen {
             }
             op.vendorExtensions.put("x-codegen-pistache-consumesJson", consumeJson);
             op.vendorExtensions.put("x-codegen-pistache-isParsingSupported", isParsingSupported);
+            for(String hdr : op.imports) {
+                if(importMapping.containsKey(hdr)) {
+                    continue;
+                }
+                additionalProperties.put("hasModelImport", true);
+            }
         }
 
         return objs;