feat(remote-feature-flag-controller): return threshold values directly and track threshold groups

[asalsys]

Summary

• BREAKING: Threshold feature flags now return the selected value directly from remoteFeatureFlags instead of a { name, value } wrapper object.
• Add optional featureFlagThresholdGroups state field (Record<string, string>) that maps feature flag names to their selected threshold group name when the selected threshold entry includes thresholdName.
• Remove normalizeThresholdValue and the legacy { name, value } wrapper fallback for unversioned threshold entries.

Motivation

Separating the processed flag value from threshold group metadata makes it easier for consumers to use threshold flags as plain values (booleans, objects, etc.) without unwrapping. The new featureFlagThresholdGroups field provides explicit access to A/B group assignment when configs opt in via thresholdName.

Migration

Consumers that previously read threshold group names from remoteFeatureFlags[flagName].name should instead:

1. Read the flag value directly from remoteFeatureFlags[flagName].
2. Read the threshold group name from featureFlagThresholdGroups[flagName] when available (only populated when the remote config provides thresholdName on the selected threshold entry).

Before:

const flag = remoteFeatureFlags.myThresholdFlag;
// { name: 'groupB', value: { enabled: true } }
const groupName = flag.name;
const value = flag.value;

After:

const value = remoteFeatureFlags.myThresholdFlag;
// { enabled: true }

const groupName = featureFlagThresholdGroups.myThresholdFlag;
// 'groupB' (only when thresholdName is configured)

Test plan

• yarn workspace @metamask/remote-feature-flag-controller run jest --no-coverage remote-feature-flag-controller.test.ts — all 53 tests pass
• Threshold flags without thresholdName return the value directly and leave featureFlagThresholdGroups empty
• Threshold flags with thresholdName return the value directly and populate featureFlagThresholdGroups
• Stale featureFlagThresholdGroups entries are cleaned up when flags are removed from the server response

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