Modified error handling in UnmarshalString to marshal non-string input into a JSON string

graphql/string.go

@@ -64,6 +64,11 @@ func UnmarshalString(v any) (string, error) {
 	case nil:
 		return "", nil
 	default:
-		return "", fmt.Errorf("%T is not a string", v)
+		payload, err := json.Marshal(v)
+		if err != nil {
+			return "", fmt.Errorf("failed to marshal value of type %T: %w", v, err)
+		}
+
+		return string(payload), nil
 	}
 }

graphql/string_test.go

@@ -31,6 +31,8 @@
 		assert.Equal(t, "true", mustUnmarshalString(t, true))
 		assert.Equal(t, "false", mustUnmarshalString(t, false))
 		assert.Equal(t, "", mustUnmarshalString(t, nil))
+		assert.Equal(t, `{"hello":"world"}`, mustUnmarshalString(t, map[string]interface{}{"hello": "world"}))
+		assert.Equal(t, `{"hello":123}`, mustUnmarshalString(t, map[string]interface{}{"hello": 123}))
 	})
 }