fix(packages/sui-segment-wrapper): improve session id assignment

[kikoruiz]

Summary

Fix GA4 session ID inconsistencies that caused "unassigned" sessions when matching events from Segment (cloud) and client-side "sui" events.

Root cause: gtag.get('session_id') returns incorrect/temporary IDs during the first hits due to a race condition between the API call and the physical cookie write.

Solution: Prioritize reading session ID directly from the GA4 cookie (segment_ga_*) as the source of truth, with API fallback.

Changes

• Add getGA4SessionIdFromCookie() helper in utils/cookies.js to read session ID from cookie using regex pattern
• Update getGoogleSessionId() to prioritize cookie value over gtag API
• Extract checkNewSession() helper to deduplicate session detection logic
• Add getGA4Data() function to expose complete GA4 context (clientId, sessionId, isNewSession)
• Export new functions: getGA4Data, getGoogleClientId, getGoogleSessionId
• Support optional googleAnalyticsCookiePrefix config (defaults to 'segment')
• Expose GA4 data in window.__GA4_DATA for debugging

Backwards Compatibility

✅ 100% backwards compatible

• All existing functions maintain same signature and behavior
• Event flow to Segment unchanged
• "sui" GA4 init events still trigger correctly
• Graceful fallbacks if cookie/localStorage unavailable

Benefits

• Consistent session IDs between client and Segment events
• Reduced "unassigned" sessions in GA4
• Better session tracking accuracy
• Improved debugging capabilities