add test for model with required fields and additionalProperties: false

Author: ctreatma

samples/client/petstore/go/model_test.go

@@ -0,0 +1,21 @@
+package main
+
+import (
+	"testing"
+
+	sw "github.com/OpenAPITools/openapi-generator/samples/client/petstore/go/go-petstore"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRequiredFieldsWithAdditionalPropertiesFalse(t *testing.T) {
+	assert := assert.New(t)
+
+	newAnimal := (sw.Animal{})
+	jsonAnimal := `{"className":"invalidAnimal","extraThing":"foo"}`
+
+	err := newAnimal.UnmarshalJSON([]byte(jsonAnimal))
+	expected := "json: unknown field \"extraThing\""
+
+	assert.ErrorContains(err, expected, "Animal should return error when missing additional fields are present")
+}