GREEN phase of TDD for Task 3:
- Implement markAsRead: resets unread count to 0, updates lastReadAt timestamp
- Implement incrementUnreadCount: safely handles missing channels
- Implement setWindowPosition: stores UI position for WindowPortal
- Export all 7 action creators from slice
- Complete sessionChatSlice with full reducer suite
All 40 tests pass including 3 integration tests:
- Complete flow validates message receiving, channel switching, window state
- Multi-channel flow validates independent unread counts per channel
- Deduplication test validates WebSocket + REST double-message scenario
Ready for async thunks in Phase 7 Plan 2.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
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