Make GlToast work in Vue 3 compat mode

What does this MR do?

Make GlToast work in Vue 3 compat mode

Fix three issues preventing toasts from rendering correctly in Vue 3 compat (and a pre-existing test ordering bug in Vue 2):

1. $slots is readonly in Vue 3 compat, so bv-toast.js assigning toast.$slots['toast-title'] silently failed. Set slots on the internal instance (toast.$.slots) as functions for Vue 3.
2. renderTitle called $createElement outside a render context, which in Vue 3 compat triggers "resolveComponent can only be used in render() or setup()" and produces broken VNodes. Changed renderTitle to return a deferred render function that is called during the slot evaluation in the component's render cycle.
3. ensureToaster only checked Wormhole.hasTarget() before creating a toaster. If the toaster DOM was removed but the Wormhole registration persisted, no new toaster was created and toasts were portaled to a detached target. Added a DOM existence check to detect and recover from stale toaster state.

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Integrations

• GitLab: Draft: GitLab integration test for https://gitl... (gitlab-org/gitlab!223742 - closed)
• CustomersDot: mr_url
• Duo UI: mr_url
• Status Page: mr_url
• Docs: mr_url
• Switchboard: https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/switchboard/-/merge_requests/3632+

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