chore: reduce handwritten Marshal/Unmarshall code for int64 type

FUTURE_COPYBARA_INTEGRATE_REVIEW=#334 from googleapis:release-please--branches--main 0f59ea9
PiperOrigin-RevId: 765272067

Author: qiaodev

type_helpers.go

@@ -0,0 +1,52 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package genai
+
+import (
+	"encoding/json"
+	"fmt"
+	"strconv"
+)
+
+type int64sFromStringSlice []int64
+
+func (s *int64sFromStringSlice) UnmarshalJSON(data []byte) error {
+	var stringSlice []string
+	if err := json.Unmarshal(data, &stringSlice); err != nil {
+		// If both attempts fail, return a more informative error
+		return fmt.Errorf("failed to unmarshal as []int64 or []string: %w", err)
+	}
+
+	// If successful as a []string, convert each element to int64
+	result := make([]int64, 0, len(stringSlice))
+	for _, str := range stringSlice {
+		val, err := strconv.ParseInt(str, 10, 64)
+		if err != nil {
+			return err // Error during string-to-int conversion
+		}
+		result = append(result, val)
+	}
+
+	*s = result
+	return nil
+}
+
+func (s int64sFromStringSlice) MarshalJSON() ([]byte, error) {
+	stringSlice := make([]string, 0, len(s))
+	for _, val := range s {
+		stringSlice = append(stringSlice, strconv.FormatInt(val, 10))
+	}
+	return json.Marshal(stringSlice)
+}