Netflux server-side idle timeout is hardcoded at 65s and kills healthy realtime sessions in throttled background tabs (server-side counterpart of XWIKI-23928)

The secondary observation (the editor stuck on "Connecting" after a disconnect, with ERROR ENOENT "Channel [...] not found") is now handled separately in XWIKI-25236.

I could reproduce it with the fix for this issue in place: it happens whenever a user who is editing alone loses the connection long enough for the WebSocket to be closed, so the server-side pings make it rarer (healthy background tabs aren't disconnected anymore) but don't prevent it. The investigation also refined the likely cause I gave in my previous comment: the ENOENT itself is harmless, the editor gets stuck because it waits forever for the auto-saver to stop while leaving the old realtime session. The details are in XWIKI-25236.