POS hooks for the Belgian blackbox v2

Commit message

[IMP] point_of_sale: add support for Belgian blackbox v2

Refactor several POS core methods into overridable hooks so that
l10n_be_pos_blackbox (v2) can intercept and extend critical flows
to implement the new blackbox requirements.

point_of_sale:
- pos_store.js: extract posBackOnline(), openCashbox(),
  getSelfOrderToPrint(), and resetCashier() as dedicated methods;
  setCashier() now returns a boolean; preSyncAllOrders() now returns
  the orders array, and the sync loop skips an order when it returns
  falsy (allows blackbox to block premature syncing); add
  orderReceiptComponent class property so the receipt component can
  be substituted by submodules.
- order_payment_validation.js: extract canPrintReceipt getter (makes
  it overridable); fix absolute import path for error_handlers; fix
  typo "occured" → "occurred".
- pos_session.py: load product.template / product.product before
  account.tax in _load_pos_data_models to satisfy the blackbox data
  dependency order; include account_move id in the invoice list
  returned by the session sales-details report.

pos_hr:
- Split setCashier() into setCashier() + setCashierUpdateSession()
  so the session-update side-effect can be called independently by
  the blackbox during clock-in/out flows. Return true from
  setCashier() consistently with the base method.

pos_loyalty:
- Override displayPrice on order lines so gift-card trigger products
  always display a positive price, even when the order is a refund.

pos_restaurant:
- Refactor mergeOrders() into _mergeOrders() + _mergeLines() private
  helpers; add getLinesToMerge() hook so blackbox can filter which
  lines participate in a merge; mergeOrders() now returns the
  destination order; extract syncRestoredOrders() for overridability;
  replace the while-loop-with-guard-counter with a plain for-of loop.

pos_self_order:
- Add orderReceiptComponent property; extract
  handleKioskSessionStatusChange() so the kiosk status-change
  behaviour can be overridden by the blackbox self-order bridge.


Task-id: 5864870
Related: odoo/enterprise#132545