[WS-3079]: Send a Piano activation event when a user is activated into an Optimizely experiment

[LukasFrm]

Resolves JIRA: https://bbc.atlassian.net/browse/WS-3079

Summary

Sends a standalone Piano/Reverb "activation" event when a user is activated into an Optimizely experiment, for both server-side and client-side experiments.

Code changes

• Add buildActivationEventModel and sendOptimizelyActivationEvent to build and guard-send the activation beacon
• Add useOptimizelyActivationEvent hook to gather tracking context and expose a send callback
• Add once-per-experiment ref dedupe to activateExperiment, useServerSide, and useClientSide

Testing

1. yarn jest src/app/hooks/useOptimizelyVariation src/app/hooks/useOptimizelyActivationEvent src/app/lib/analyticsUtils src/app/components/ATIAnalytics
2. Force a server-side experiment via mvt-{experimentName}: {variation} header, confirm one activation event fires, no duplicates on re-render
3. Trigger a client-side decision, confirm same behaviour; no event for off/disabled/ineligible users
4. Block the Reverb script and confirm failure is logged, not thrown

Useful Links

• Coding Standards
• Repository use guidelines