Fix Duo panel identity verification empty state click tracking

What does this MR do and why?

The "Verify my account" button in the Duo panel identity verification empty state (ee/app/assets/javascripts/ai/duo_agentic_chat/components/identity_verification_empty_state.vue) still used the legacy click-tracking pattern: data-event-tracking="click_link" with the specific event name only in data-event-label. That pattern only ever sends the generic click_link Snowplow event — the backend rejects it as an unknown event, and on instances with gl.onlySendDuoEvents it is dropped entirely.

This MR binds data-event-tracking directly to the specific internal event click_duo_agentic_identity_verification_empty_state_verify_account (the constant already exists in DUO_PANEL_EMPTY_STATE_EVENTS). It fixes the same legacy pattern on three sibling elements: the "Learn more" link in access_denied_empty_state.vue, and the "Start a Free Trial" and "Learn more" buttons in start_trial_empty_state.vue. Each now binds data-event-tracking to its specific event constant (CLICK_LEARN_MORE or CLICK_START_TRIAL). The event definitions for these already exist. The MR also:

• Adds the event definition ee/config/events/click_duo_agentic_identity_verification_empty_state_verify_account.yml (mirroring the existing view_duo_agentic_identity_verification_empty_state definition).
• Adds a Jest spec asserting the click fires the correct event via bindInternalEventDocument / triggerEvent.

References

• Follow-up to !257025 (merged)
• Addresses review feedback: !257025 (comment 3902364918)

How to set up and validate locally

1. Run the spec:

   yarn jest ee/spec/frontend/ai/duo_agentic_chat/components/identity_verification_empty_state_spec.js

2. Confirm the "Verify my account" button now renders data-event-tracking="click_duo_agentic_identity_verification_empty_state_verify_account" (and no data-event-label).

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