[16.0][FIX] pms: check room capacity when the room of a reservation changes

[DarioLodeiros]

Problem

The room capacity check used to be a constraint on pms.reservation. It was
turned into a manual check called from create() and from write() only when
adults is in vals, to avoid false positives in intermediate states where the
extra bed services of the vals are not accessible yet.

As a side effect, no room change validates the capacity any more:

• writing room_id on the reservation lines (planning drag&drop, room swap,
  split/join/swap wizard), and
• changing preferred_room_id on the reservation, whose recompute of the room
  of the lines does not go through pms.reservation.line.write(),

both allow moving a reservation to a room that cannot hold its guests (e.g. a
2 pax reservation into a single room), with the overoccupancy that follows.

Solution

Check the capacity of the new room on both paths, in a new
pms.reservation.line._check_room_capacity().

To keep away from the false positives that motivated the manual check, it uses
the maximum capacity of the room (capacity + extra_beds_allowed) instead
of the extra beds actually sold, so a room change is never blocked because the
extra bed service is not created yet. The check can be skipped with the
avoid_capacity_check context key for massive or automated changes.

Tests

Four tests in test_pms_reservation_line.py: the room change is blocked both
by line and by preferred room, it is allowed when the destination room has an
allowed extra bed, and the context key skips it.

@commitsun

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