feat(browser-sdk/react-sdk/vue-sdk): end user opt in sdk

[roncohen]

Add end-user opt-in helpers for listing opt-in-enabled flags and setting whether the current user or company has opted into a flag.

[Copilot]

Pull request overview

This PR introduces end-user opt-in support across the Browser SDK, plus React/Vue convenience hooks/composables and documentation so applications can list opt-in-enabled flags and toggle opt-in membership for the current user or company.

Changes:

• Add Browser SDK APIs/types for opt-in (getOptInFlags, setOptIn, OptInFlag, SetOptInOptions) and persist/parse opt-in metadata from evaluated flags.
• Add React hooks (useOptInFlags, useSetOptIn) and Vue composables (useOptInFlags, useSetOptIn) with corresponding usage tests.
• Improve Browser SDK flag refresh concurrency handling/rate limiting and add regression tests for out-of-order refreshes/context changes.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
packages/vue-sdk/test/usage.test.ts	Adds Vue tests covering opt-in composables behavior and delegation.
packages/vue-sdk/src/index.ts	Re-exports new opt-in composables and opt-in related types.
packages/vue-sdk/src/hooks.ts	Implements useOptInFlags and useSetOptIn composables.
packages/vue-sdk/README.md	Documents Vue opt-in composables with an example UI pattern.
packages/react-sdk/test/usage.test.tsx	Adds React hook tests for opt-in listing/updates and setOptIn delegation.
packages/react-sdk/src/index.tsx	Implements and exports React opt-in hooks and re-exports opt-in types.
packages/react-sdk/README.md	Documents React opt-in hooks with example usage and semantics.
packages/browser-sdk/test/flags.test.ts	Adds tests for opt-in parsing and refresh concurrency/rate limiting; imports validateFlagsResponse.
packages/browser-sdk/test/deferred.ts	Introduces a small deferred helper used by concurrency tests.
packages/browser-sdk/test/client.test.ts	Adds end-to-end tests for opt-in flows and related state/concurrency behavior.
packages/browser-sdk/src/index.ts	Exports new opt-in-related types from the Browser SDK entrypoint.
packages/browser-sdk/src/flag/flags.ts	Adds opt-in flag types and improves refresh behavior for concurrent/out-of-order responses.
packages/browser-sdk/src/flag/flagCache.ts	Extends API flag parsing to include opt-in metadata from responses/cached values.
packages/browser-sdk/src/client.ts	Implements getOptInFlags and setOptIn, plus context-loading finalization for superseded updates.
packages/browser-sdk/README.md	Documents Browser SDK opt-in API usage and expected semantics.
.changeset/end-user-opt-in-sdk.md	Publishes minor versions for browser/react/vue SDKs for this feature.

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