feat: Add generic useABTest hook for A/B testing

[bfullam]

Description

Adds a useABTest hook that provides a simple, type-safe interface for A/B testing using LaunchDarkly feature flags. The hook reads the assigned variant from RemoteFeatureFlagController (which handles bucketing via SHA256 hash of metametricsId + flagName) and maps it to variant data defined by the caller.

Why: We need a standardized, reusable pattern for running A/B tests without requiring per-test boilerplate or Segment schema changes for each experiment.

What's included:

• Generic useABTest hook with TypeScript support
• Handles both object format { name } from controller and legacy string format
• Comprehensive unit tests (26 test cases)
• Documentation covering LD setup, analytics integration, and bucketing explanation

Changelog

CHANGELOG entry: null

Related issues

Fixes: N/A - Infrastructure for future A/B testing

Manual testing steps

1. Hook returns correct variant when flag is set: Configure LD to return { name: "treatment" } → hook returns treatment variant data, isActive is true
2. Hook falls back to control when flag is not set: No LD flag configured → hook returns control variant data, isActive is false
3. Analytics tracking includes ab_tests property: With active test, SWAP_PAGE_VIEWED event includes ab_tests: { select_amount_text: "treatment" }

Screenshots/Recordings

Before

N/A - New feature

After

N/A - No UI changes; hook is internal infrastructure. First experiment (select amount text) requires LD flag to be configured to observe behavior difference.

Pre-merge author checklist

• I've followed MetaMask Contributor Docs and MetaMask Mobile Coding Standards.
• I've completed the PR template to the best of my ability
• I've included tests if applicable
• I've documented my code using JSDoc format if applicable
• I've applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

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Note

Low Risk
New hook + docs/tests with no changes to existing runtime flows unless adopted by callers; primary risk is incorrect variant selection/activation logic when teams start using it.

Overview
Adds a new generic useABTest React hook that maps a remote feature-flag value to locally-defined variant data, exposing { variant, variantName, isActive } with a guaranteed control fallback and protection against prototype-key matches.

The hook supports both the controller’s { name } A/B-test object format and a legacy string flag value format, is exported via app/hooks/index.ts, and is backed by extensive unit tests plus a new docs/ab-testing.md guide covering LaunchDarkly configuration and analytics tracking conventions.

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[github-actions]

🔍 Smart E2E Test Selection

• Selected E2E tags: None (no tests recommended)
• Selected Performance tags: None (no tests recommended)
• Risk Level: low
• AI Confidence: 95%

click to see 🤖 AI reasoning details

E2E Test Selection:
This PR introduces a new A/B testing framework consisting of:

1. New useABTest hook (app/hooks/useABTest.ts) - A generic React hook for A/B testing that reads feature flags from the RemoteFeatureFlagController and maps them to variant data
2. Unit tests (app/hooks/useABTest.test.ts) - Comprehensive test coverage with 465 lines of tests
3. Hooks index (app/hooks/index.ts) - Centralized exports for hooks
4. Documentation (docs/ab-testing.md) - Framework documentation

Why no E2E tests are needed:

• All files are new additions - no existing code is modified
• The useABTest hook is not used anywhere in the application yet (verified via grep search - only found in its own files)
• No UI components are affected by these changes
• No existing user flows or E2E test scenarios could be impacted
• The hook relies on existing stable infrastructure (selectRemoteFeatureFlags selector from app/selectors/featureFlagController)
• The comprehensive unit tests (465 lines) provide adequate validation for this infrastructure code

This is purely additive infrastructure code that enables future A/B testing capabilities but doesn't change any current app behavior.

Performance Test Selection:
No performance tests needed. This PR adds a new useABTest hook that is not yet used anywhere in the application. The hook is a simple wrapper around existing Redux selectors (selectRemoteFeatureFlags) with minimal logic - just reading a flag value and mapping it to variant data. There are no UI rendering changes, no data loading changes, no list rendering, and no impact on app startup or critical user flows. The hook's performance characteristics are trivial (single useSelector call with simple object lookup).

View GitHub Actions results

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sahar-fehri]

Nit: wonder if it makes sense that the hook takes an optional default variant value?
In case there are cases where the default value is "treatment" instead of "control" ?

[sahar-fehri]

LGTM