test(types): add unit test for JSON unmarshalling and validation

- Introduced a new test file for JSON handling to ensure that unmarshalling correctly copies input data.
- Validated that the stored JSON remains intact after input mutation, and confirmed successful marshalling of the output.
- This test enhances the reliability of JSON operations within the types package.

internal/types/json.go

@@ -59,7 +59,9 @@ func (j *JSON) UnmarshalJSON(data []byte) error {
 	if j == nil {
 		return errors.New("JSON: UnmarshalJSON on nil pointer")
 	}
-	*j = JSON(data)
+	copied := make([]byte, len(data))
+	copy(copied, data)
+	*j = JSON(copied)
 	return nil
 }
 

internal/types/json_test.go

@@ -0,0 +1,37 @@
+package types
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestJSONUnmarshalCopiesInput(t *testing.T) {
+	type wrapper struct {
+		Value JSON `json:"value"`
+	}
+
+	input := []byte(`{"value":{"generated_questions":[{"id":"q1","question":"今晚吃啥"}]}}`)
+
+	var got wrapper
+	if err := json.Unmarshal(input, &got); err != nil {
+		t.Fatalf("unmarshal failed: %v", err)
+	}
+
+	// Simulate decoder buffer reuse/caller mutation after UnmarshalJSON returns.
+	for i := range input {
+		input[i] = 'x'
+	}
+
+	if !json.Valid(got.Value) {
+		t.Fatalf("stored JSON became invalid after input mutation: %q", string(got.Value))
+	}
+
+	marshaled, err := json.Marshal(got)
+	if err != nil {
+		t.Fatalf("marshal failed: %v", err)
+	}
+
+	if len(marshaled) == 0 {
+		t.Fatal("expected marshaled output")
+	}
+}