[IMP] point_of_sale: enable combo products

Products can be created directly from the `pos.combo` view.
Before this PR, such products would not be automatically available in
the POS app.

This PR makes it so products linked to combos are available in POS by
default.

We also introduce an additional check that makes it so products associated
to combos cannot be disabled in the pos.

In this commit we also make it so the `Office Combo` product from the pos
demo data is in the `Desks` `pos_category`.

closes odoo/odoo#133359

Task: 3481899
Signed-off-by: Joseph Caburnay (jcb) <jcb@odoo.com>

addons/point_of_sale/data/point_of_sale_demo.xml

@@ -464,6 +464,7 @@
           <field name="name">Office combo</field>
           <field name="type">combo</field>
           <field name="categ_id" ref="product.product_category_5"/>
+          <field name="pos_categ_ids" eval="[(6, 0, [ref('pos_category_desks')])]"/>
           <field name="uom_id" ref="uom.product_uom_unit"/>
           <field name="uom_po_id" ref="uom.product_uom_unit"/>
           <field name="image_1920" type="base64" file="point_of_sale/static/img/office_combo.png"/>

addons/point_of_sale/models/product.py

@@ -35,6 +35,13 @@ class ProductTemplate(models.Model):
         if not self.sale_ok:
             self.available_in_pos = False
 
+    @api.constrains('available_in_pos')
+    def _check_combo_inclusions(self):
+        for product in self:
+            if not product.available_in_pos:
+                combo_name = self.env['pos.combo.line'].search([('product_id', 'in', product.product_variant_ids.ids)], limit=1).combo_id.name
+                if combo_name:
+                    raise UserError(_('You must first remove this product from the %s combo') % combo_name)
 
 class ProductProduct(models.Model):
     _inherit = 'product.product'

addons/point_of_sale/views/pos_combo_view.xml

@@ -13,7 +13,7 @@
                 </div>
                 <field name="combo_line_ids">
                     <tree editable="bottom">
-                        <field name="product_id"/>
+                        <field name="product_id" context="{'default_available_in_pos': True}"/>
                         <field name="combo_price"/>
                         <field name="lst_price"/>
                     </tree>