fix(amplitude-session): fix persistence model, reset(), inline helpers

[abueide]

Summary

• Stop persisting eventSessionId — it's a transient value bootstrapped from sessionId on load
• Batch persistence with AsyncStorage.multiSet in performSessionTransition to prevent partial state on crash
• Remove sessionId setter's AsyncStorage.setItem — persistence now only happens in performSessionTransition and reset()
• Fix reset() to fire session_end before clearing state, use multiRemove for atomic cleanup
• Inline endSession/trackSessionStart into performSessionTransition (single call site each)

Context

PR 4/4 in the fix stack. Depends on #1167. After this PR, the final source has no console.log, no resetPending, and no EVENT_SESSION_ID_KEY.

Test plan

• Add test: eventSessionId is NOT persisted to AsyncStorage
• Add test: eventSessionId is bootstrapped from sessionId on load
• Add test: multiSet used for session transition persistence
• Add test: reset() fires session_end and uses multiRemove
• Add test: reset() does NOT fire session_end when no active session
• yarn jest — 66 suites, 366 tests pass, no regressions

🤖 Generated with Claude Code

[abueide]

Superseded by a single combined PR.