test(expo): add behavioral native client sync suite

[mikepitre]

Description

Adds an engine-agnostic behavioral test suite for the @clerk/expo JS <-> native client sync, so it can gate the upcoming replacement of the sync engine (native owns the device token, payload-free invalidations in both directions).

The suite lives in packages/expo/src/provider/__tests__/nativeClientSyncBehavior/ and asserts only user-observable outcomes: the JS signed-in state and active session, the native token and client view, request and refresh counts, the absence of signed-out emissions during session switches, and errors surfaced through the sync barrier. It is backed by:

• FakeClerkServer: one Frontend API shared by both SDKs, with clients keyed by device token.
• FakeNativeClerk: the native SDK's persisted token, its client view, and native-originated actions and change notifications.
• FakeClerk: a stand-in for the clerk-js instance returned by getClerkInstance, using the same surface the existing ClerkProvider tests mock. It talks to the fake Frontend API through the token cache it is built with, like createClerkInstance.
• currentEngineAdapter: renderWithNativeSync(scenario) for today's engine. It keeps today's native module contract (configure, getClientToken, syncClientStateFromJs, clerkNativeClientChanged payloads and sourceId tags) out of the tests. To run the suite against a new engine, only the export in index.ts needs to point at a new adapter.

Two behaviors the current engine does not meet yet are kept as it.fails under "known gaps in the current engine". They should flip to it once the new engine lands:

• JS starting with a stale device token replaces native's signed-in token, because configure adopts the JS bearer token.
• If JS and native rotate the device token at the same time, the two sides end up on different tokens.

The existing ClerkProvider.nativeClientSync and coordinator tests are unchanged.

Checklist

• pnpm test runs as expected.
• pnpm build runs as expected.
• (If applicable) JSDoc comments have been added or updated for any package exports
• (If applicable) Documentation has been updated

Type of change

• 🐛 Bug fix
• 🌟 New feature
• 🔨 Breaking change
• 📖 Refactoring / dependency upgrade / documentation
• other: tests

🤖 Generated with Claude Code

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🦋 Changeset detected

Latest commit: 2e31617

The changes in this PR will be included in the next version bump.

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Project	Deployment	Actions	Updated
clerk-js-sandbox	Ready	Preview	Oct 2, 2026 4:05pm UTC
swingset	Ready	Preview	Oct 2, 2026 4:05pm UTC

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