Do not clear active_tray on transient AMS empty/unknown/unavailable states

[Choleriker]

LAN-only mode: do not clear active_tray on transient AMS empty/unknown/unavailable states.

In LAN-only mode ha-bambulab sometimes reports AMS tray states as unknown/unavailable/empty during reconnect/resync. SpoolmanSync should not clear Spoolman extra.active_tray based on those transient states.

Requested behavior:

• keep existing assignment on unknown/unavailable
• optionally debounce empty state, e.g. only clear after 2–5 minutes
• add setting: "Never auto-clear tray assignments"

[Choleriker]

Entity:
sensor._ams_1_slot_3

Observed behavior:
Random transient state changes:
PLA -> Empty -> PLA
within ~4-5 seconds in LAN-only mode.

This causes SpoolmanSync to remove the spool assignment although the spool never changed physically.

This appears to be a temporary AMS/ha-bambulab state glitch, not a real empty tray event.

Requested fix:
Do not immediately clear assignments on transient Empty states.
Use debounce/timer or optional "never auto-clear assignments" setting.