Use _merged_session_messages_for_display for is_messaging_session even in the
metadata-only (messages=0) path. This ensures message_count and last_message_at
match the full load path for Telegram / external messaging sessions that have
stitched or duplicate rows in state.db + sidecar.
Prevents spurious refresh loops, scroll resets, and open panel closures when
resuming cross-surface sessions in the WebUI.
No impact on CLI, non-messaging, or full-message paths. All 580 session tests pass.
Fixes the root cause identified in the SessionDB / render interaction changes.
george-andraws