feat(addons): add abTestAssignment addon

[jrosendahl-opt]

Summary

Moved all of the client side A/B test management to a single add-on.

##Changes

• Adds lib/edge/abTest.ts with determineABTest extracted from targeting.ts — keeps the core assignment logic in the edge layer where targeting can use it without depending on addons
• Adds lib/addons/abTestAssignment.ts with setupAB: manages client-side A/B variant assignment with localStorage stickiness, sessionStorage override, and Prebid bid stamping via applyToAuctionEvent / setHooks
• Removes determineABTest and the unused ABTestConfig re-export from targeting.ts
• Moves determineABTest tests from targeting.test.js into abTestAssignment.test.ts
• Adds lib/addons/abTestAssignment.md with usage docs

[mosherBT]

The flow here needs to be more heavily commented. Just to explain first visit vs cache and things that are not immediately obvious on reading code

[jrosendahl-opt]

Done

[mosherBT]

I think we need to accept this from the URL as well optableControlGroup

[jrosendahl-opt]

This should not be a mistake devs can make, I pulled out flags into a separate function to manage them. Modules should always call getFlags() from not on.

[mosherBT]

We are missing cache clearing based on split test. I think that will bite us so we need to address it.

Something like this

// Control group: clear cached EIDs so RTD/PPID/targetingFromCache serve nothing
if (isControlGroup) {
  try {
    localStorage.removeItem('OPTABLE_RESOLVED');
    Object.keys(localStorage)
      .filter((k) => k.startsWith('OPTABLE_TARGETING_'))
      .forEach((k) => localStorage.removeItem(k));
  } catch {
    /* ignore */
  }
}

[jrosendahl-opt]

Fixed

[jplaroche]

The object.keys portion (including the 2 next lines), can be replaced with {SDK}.targetingClearCache();

[jrosendahl-opt]

Also fixed.