Surveys: Support linkedFlagVariant for targeting specific feature flag variants

Summary

The Flutter SDK does not support linkedFlagVariant for surveys. Since Flutter wraps the native iOS and Android SDKs, this depends on those SDKs implementing the feature first.

Current State

Flutter currently requires:

• iOS SDK: >= 3.38.0 (does not support linkedFlagVariant)
• Android SDK: >= 3.25.0 (does not support linkedFlagVariant)

The native SDKs only check if a linked feature flag is enabled (isFeatureEnabled), but do not support targeting users in a specific variant of that flag.

Expected Behavior

When a survey has conditions.linkedFlagVariant set (e.g., "variant-a"), the SDK should:

1. Get the actual flag value: flags[linkedFlagKey]
2. Compare it to the variant: flagValue === linkedFlagVariant || linkedFlagVariant === "any"

Dependencies

This feature requires implementation in the native SDKs first:

• iOS: https://github.com/PostHog/posthog-ios/issues/445
• Android: https://github.com/PostHog/posthog-android/issues/388

Once implemented, Flutter will need to bump its minimum native SDK dependency versions.

Reference Implementation

See posthog-react-native: packages/react-native/src/surveys/getActiveMatchingSurveys.ts lines 77-83

Tracking

This is tracked in the survey SDK feature parity issue: https://github.com/PostHog/posthog/issues/45658

Start with the reference implementation in packages/react-native/src/surveys/getActiveMatchingSurveys.ts, lines 77-83, then review the linked iOS and Android issues and the Flutter native SDK dependency versions. Done means the native SDKs support linkedFlagVariant and Flutter raises its minimum versions so surveys can target a specific variant or any variant.