feat: surface multivariate variant key and use it as the exposure value

[Zaimwa9]

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• I have read the Contributing Guide.
• I have added information to docs/ if required so people know about the feature.
• I have filled in the "Changes" section below.
• I have filled in the "How did you test this code" section below.

Changes

• IFlagsmithFeature.variant — selected variant's key, "control" for the control bucket, omitted otherwise.
• getExperimentFlag records the variant as the $flag_exposure value; flags without a variant record no exposure.
• useExperiment re-fires the exposure when the variant changes.

How did you test this code?

Unit tests: variant parsing, exposure values, variant-change re-exposure

[gemini-code-assist]

Code Review

This pull request introduces support for multivariate flag variants in the Flagsmith SDK. It updates the core SDK to extract and store the variant property on flags, uses the variant value instead of the flag value for experiment exposure tracking, and updates the React hooks (useFlags, useExperiment) to handle and trigger re-renders on variant changes. Additionally, types and tests are updated to support this new feature. Feedback from the review highlights two main areas of improvement: first, in flagsmith-core.ts, if a flag is missing (null), the current logic logs a misleading warning about a missing variant, so it is recommended to check if the flag exists before checking for its variant; second, in react.tsx, the use of .map() to mutate an object is an anti-pattern and should be refactored to use .forEach() for better clarity and performance.