Follow-up: Fix spec violations in add_edit_schedule_modal_spec.js

What does this MR do and why?

Following up on:

• !122482 (comment 1418773592)
• !122482 (comment 1418773595)

1. Replace createLocalVue with Vue.use(VueApollo);
2. Cover different UI states with new checks: buttons, alert dismissal, copy changing for create/edit mode

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Checkout the branch locally and run yarn jest ee/spec/frontend/oncall_schedule/add_edit_schedule_modal_spec.js

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #413340 (closed)