Show alert in CI Variable drawer

What does this MR do and why?

Resolves #434666 (closed).

When the user adds or edits a CI variable, the drawer closes and a toast notification is shown as feedback. In this MR, we are removing the toast notification and instead show the alert inside the drawer, which will no longer close after the user successfully creates or updates a CI variable. This alert will also show error messages from these GraphQL mutations.

Note that for deleting CI variables, the toast notification will still appear.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After
before	after

Delete variable behavior is not affected

delete_variable

How to set up and validate locally

1. In a group or project, visit the CI/CD settings. Settings > CI/CD
2. Expand the Variables section.
3. Add or edit a CI variable and verify that:
   • The drawer doesn't close after a successful a mutation
   • The alert message shows up inside the drawer
   • There is no toast notification