fix(amplitude-session): fix 0-second sessions on iOS

[abueide]

Summary

Fix AmplitudeSessionPlugin producing 0-second sessions (session_start + session_end at the same timestamp) on iOS. Addresses customer escalation from v0.4.2 rewrite (PR #1104).

Root cause: The plugin ignored iOS's inactive AppState. On iOS, the lifecycle is active → inactive → background, but the plugin only handled background, so the active → inactive transition was a no-op. This meant lastEventTime was never updated on backgrounding, causing immediate session expiration when the app resumed.

Changes

Handle iOS inactive AppState (primary fix)

Track _previousAppState and treat inactive the same as background for session timeout, matching the core library's pattern at analytics.ts:771. Without this, every iOS background/foreground cycle produces a spurious session_end + session_start.

Await onForeground

onForeground was fire-and-forget, so the session transition could race with incoming events. Now awaited.

Replace resetPending with _sessionTransition promise

The resetPending flag was cleared asynchronously via the event pipeline when session_start flowed back through track(). Between the flag being set and cleared, a race window existed where concurrent execute() calls could skip the transition check. The promise lets concurrent callers await the same in-flight transition.

Remove dual-ID concept (eventSessionId)

The v0.4.2 rewrite added eventSessionId as a separate ID for event enrichment, intended to let events keep the old session ID during transitions. But track() overwrote it when session events flowed back through the pipeline — by that point the transition had already completed, so eventSessionId held the new value anyway. With the transition promise serializing access (no event can be enriched mid-transition), the second ID is unnecessary. Removed _eventSessionId, EVENT_SESSION_ID_KEY, and related getter/setter.

Fix track() to preserve existing session_id

When session_start/session_end events flow back through track(), preserve the session_id already set by performSessionTransition instead of overwriting it with the current sessionId.

Fix reset() to fire session_end

reset() now fires session_end before clearing state, matching the Swift SDK behavior. Uses multiRemove for atomic cleanup.

Batch persistence

Use AsyncStorage.multiSet in transitions instead of individual fire-and-forget setItem calls. Removes sessionId setter's persistence side-effect (now only persisted in performSessionTransition and reset()).

Cleanup

Remove 3 unconditional console.log debug statements and a dead //await this.saveSessionData() comment left from the v0.4.2 rewrite.

Test plan

• All 23 plugin tests pass (including new concurrency, app state, and persistence tests)
• Full test suite passes (66 suites, 363 tests, 0 failures)
• Manual verification on iOS device: background/foreground cycles no longer produce 0-second sessions

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