[GO] Add assert constraints checks for complex types in the model tem…

…plate (OpenAPITools#18654)

* [GO] Add assert constraints checks for complex types in the model template

* [GO] Update samples

* [GO] revert AssertRecurseInterface naming

modules/openapi-generator/src/main/resources/go-server/model.mustache

@@ -174,6 +174,61 @@ func Assert{{classname}}Constraints(obj {{classname}}) error {
 		return &ParsingError{Param: "{{name}}", Err: errors.New(errMsgMaxValueConstraint)}
 	}
 {{/maximum}}
+{{#isNullable}}
+{{#isModel}}
+    if obj.{{name}} != nil {
+{{/isModel}}
+{{#isArray}}
+{{#items.isModel}}
+    if obj.{{name}} != nil {
+{{/items.isModel}}
+{{/isArray}}
+{{/isNullable}}
+{{#isModel}}
+{{#isNullable}}     {{/isNullable}}	if err := Assert{{baseType}}Constraints({{#isNullable}}*{{/isNullable}}obj.{{name}}); err != nil {
+{{#isNullable}}     {{/isNullable}}		return err
+{{#isNullable}}     {{/isNullable}}	}
+{{/isModel}}
+{{#isArray}}
+{{#items.isModel}}
+{{#isNullable}}     {{/isNullable}}	for _, el := range {{#isNullable}}*{{/isNullable}}obj.{{name}} {
+{{#isNullable}}     {{/isNullable}}		if err := Assert{{items.baseType}}Constraints(el); err != nil {
+{{#isNullable}}     {{/isNullable}}			return err
+{{#isNullable}}     {{/isNullable}}		}
+{{#isNullable}}     {{/isNullable}}	}
+{{/items.isModel}}
+{{^items.isModel}}
+{{#mostInnerItems.isModel}}
+{{^mostInnerItems.isPrimitiveType}}
+{{#isNullable}}
+    if obj.{{name}} != nil {
+{{/isNullable}}
+{{#isNullable}}     {{/isNullable}}	if err := AssertRecurseInterfaceRequired({{#isNullable}}*{{/isNullable}}obj.{{name}}, Assert{{mostInnerItems.dataType}}Constraints); err != nil {
+{{#isNullable}}     {{/isNullable}}		return err
+{{#isNullable}}     {{/isNullable}}	}
+{{/mostInnerItems.isPrimitiveType}}
+{{/mostInnerItems.isModel}}
+{{/items.isModel}}
+{{/isArray}}
+{{#isNullable}}
+{{#isModel}}
+    }
+{{/isModel}}
+{{#isArray}}
+{{#items.isModel}}
+    }
+{{/items.isModel}}
+{{^items.isModel}}
+{{#mostInnerItems.isModel}}
+{{^mostInnerItems.isPrimitiveType}}
+{{#isNullable}}
+    }
+{{/isNullable}}
+{{/mostInnerItems.isPrimitiveType}}
+{{/mostInnerItems.isModel}}
+{{/items.isModel}}
+{{/isArray}}
+{{/isNullable}}
 {{/Vars}}
 	return nil
 }{{/model}}{{/models}}