Explicitly define exposed properties in popovers.vue

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What does this MR do and why?

Explicitly exposes addPopovers and dispose from the popovers.vue component and removes the component from the vue-no-unused-properties.mjs exclude list.

• addPopovers is used outside this component at app/assets/javascripts/popovers/index.js#L29.
• dispose is used outside this component at app/assets/javascripts/popovers/index.js#L50

The disconnect test of the popovers.vue component is refactored to use mocking mechanisms that don't rely on the underlying observer property of the component.

References

• Related issue: Address `vue/no-unused-properties` violations i... (#525757)
• Related epic: Address `vue/no-unused-properties` offenses (&17180)

How to set up and validate locally

1. Verify that app/assets/javascripts/popovers/components/popovers.vue is not in the .eslint_todo/vue-no-unused-properties.mjs list
2. Run eslint app/assets/javascripts/popovers/components/popovers.vue

MR acceptance checklist

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