feat(ab-test): add selectABTest addon for split test selection and persistence

[MO-Thibault]

Summary

Extracts A/B test selection logic that lives duplicated across wrapper bundles into a reusable addon at lib/addons/prototypes/ab-test.js.

Exports:

• calculateTrafficPercentages(abTests) — fills missing trafficPercentage values by distributing the remaining % evenly
• determineABTest(abTests) — weighted random bucket selection (mirrors lib/edge/targeting.ts)
• selectABTest(abTests) — full selection flow: forced overrides → exclude filter → random selection → persist to sessionStorage.OPTABLE_SPLIT_TEST

sessionStorage override keys (value "1" or "true" to activate):

• optableInclude<ID> / optableResolve<ID> — force a specific test
• optableExclude<ID> / optableInclude<ID>=false — exclude a test from random selection

OptablePrebidAnalytics accepts config.selectedTest (the result of selectABTest) and includes selectedABTest in every auction_processed witness payload.

Migration for wrappers

import { selectABTest } from '@optable/web-sdk/lib/dist/addons/prototypes/ab-test';

const selectedTest = selectABTest([
  { id: 'none', trafficPercentage: 5 },
  { id: 'all' }, // auto-filled to 95%
]);

// side effects (RTD gating, localStorage) stay in wrapper
window.optable.disabled = selectedTest?.id === 'none';
localStorage.setItem('OPTABLE_ENABLED', !window.optable.disabled);

new OptablePrebidAnalytics(optableInstance, {
  analytics: true,
  tenant: 'aditude',
  selectedTest,
});

Wrappers can drop: calculateTrafficPercentages, determineABTest, splitTestSelection, and the customAnalytics.selectedABTest block.

Test plan

• selectABTest returns null for empty/null input
• Traffic percentages auto-fill correctly when some tests omit trafficPercentage
• Forced override via optableIncludeNONE=1 selects the none test regardless of random
• Exclude override via optableExcludeNONE=1 removes none from random pool
• Result persisted to sessionStorage.OPTABLE_SPLIT_TEST
• selectedABTest appears in witness payload when config.selectedTest is set