feat(rest-api-client): add elementId to label and border fields of form layout

[y-okady]

Why

The February 2026 kintone update added an element ID to label and border fields. getFormLayout and updateFormLayout now return and accept elementId for LABEL and HR, the same way they already do for SPACER.

What

• Add a required elementId property to KintoneFormLayout.Field.Label and KintoneFormLayout.Field.HR, matching Spacer.
• Document elementId for space, label, and border fields in getFormLayout and updateFormLayout.
• Include elementId on the label field in the form layout request fixtures.

How to test

Access elementId on a variable of type KintoneFormLayout.Field.Label and confirm that TypeScript reports no type error.

import type { KintoneFormLayout } from "@kintone/rest-api-client";
declare const label: KintoneFormLayout.Field.Label;
const elementId: string = label.elementId;

Checklist

• Read CONTRIBUTING.md
• Updated documentation if it is required.
• Added tests if it is required.
• Passed pnpm lint and pnpm test on the root directory.