Track tab clicks on the trial Credits Dashboard views

What does this MR do and why?

Only the paid non-trial Credits Dashboard (app.vue) tracked its tab clicks. free_tier_trial_app.vue and paid_tier_trial_period_view.vue rendered the same tabs without the click handlers, so once most dashboard traffic moved to trial users the click_usage_overview_tab_credits_dashboard and click_usage_by_user_tab_credits_dashboard events stopped arriving altogether.

This MR emits the same events from the two trial components, reusing the definitions already in ee/config/events. paid_tier_trial_period_view.vue gains the InternalEvents mixin; free_tier_trial_app.vue already had it. The free tier view has one tab (Usage by user); the paid trial view has two (Usage overview, Usage by user). Specs cover every tab, and they fail without the component change.

References

• Resolves #618758 (closed)

Differences

Before: Clicking a tab on the free-tier trial or paid-tier trial dashboard emits no internal event.

After: Each tab click emits the same event app.vue already emits for that tab.

How to set up and validate locally

1. Open the Credits Dashboard as a trial group (Settings > Usage quotas > GitLab Credits).
2. With the Snowplow Micro or the Network tab open, click Usage by user.
3. On master no click_usage_by_user_tab_credits_dashboard event is sent; with this MR it is.

MR acceptance checklist

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