Waiting list: group product choices by category (#6006)

* Group waiting list product choices by category

Use optgroups to group products by category in the waiting list selection
dropdown.

Products are normally separated in the UI by category grouping, but this
context is lost in the waiting list form. When multiple products share the
same name, this can make it difficult for customers to distinguish between
them.

* Add tests for waiting list initial selection with optgroups

Verify that the initial product selection (via `?item=` and `?var=`
query parameters) works correctly when choices are grouped by category
into `<optgroup>`s. Covers both plain items and items with variations.

src/pretix/presale/views/waiting.py

@@ -66,22 +66,27 @@ class WaitingView(EventViewMixin, FormView):
                 if customer else None
             ),
         )
-        choices = []
+        groups = {}
         for i in items:
             if not i.allow_waitinglist:
                 continue
 
+            category_name = str(i.category.name) if i.category else ''
+            group = groups.setdefault(category_name, [])
+
             if i.has_variations:
                 for v in i.available_variations:
                     if v.cached_availability[0] == Quota.AVAILABILITY_OK:
                         continue
-                    choices.append((f'{i.pk}-{v.pk}', f'{i.name} – {v.value}'))
+                    group.append((f'{i.pk}-{v.pk}', f'{i.name} – {v.value}'))
 
             else:
                 if i.cached_availability[0] == Quota.AVAILABILITY_OK:
                     continue
-                choices.append((f'{i.pk}', f'{i.name}'))
-        return choices
+                group.append((f'{i.pk}', f'{i.name}'))
+
+        # Remove categories where all items were available (no waiting list choices)
+        return [(cat, choices) for cat, choices in groups.items() if choices]
 
     def get_form_kwargs(self):
         kwargs = super().get_form_kwargs()