[FIX] website_sale_aplicoop: three defects in the group order slug

- create(): slugs were only checked against the database, so records created
  in the same batch (duplicating several orders from the list view, importing
  rows sharing a name) collided on group_order_slug_uniq. _generate_unique_slug
  now also skips the slugs already handed out in the batch.
- _redirect_to_slug_url(): `post` is passed as a dict instead of splatted, so a
  query parameter named `suffix` no longer binds to the keyword argument of the
  same name (HTTP 500 on /eskaera/<id>/checkout?suffix=x, and a corrupted path
  on the shop route).
- post-migrate: the backfill runs with tracking_disable, `slug` is tracked and
  the upgrade posted a chatter message on every pre-existing order.

Tests for the two reachable cases: batch create and query parameters kept
across the legacy redirect.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

website_sale_aplicoop/controllers/website_sale.py

@@ -718,8 +718,12 @@ class AplicoopWebsiteSale(WebsiteSale):
             return "/eskaera"
         return f"/eskaera/{group_order.slug}{suffix}"
 
-    def _redirect_to_slug_url(self, order_id, suffix="", **post):
-        """Send a legacy /eskaera/<id> URL to its /eskaera/<slug> equivalent."""
+    def _redirect_to_slug_url(self, order_id, post, suffix=""):
+        """Send a legacy /eskaera/<id> URL to its /eskaera/<slug> equivalent.
+
+        `post` is passed as a plain dict, not splatted: a query parameter named
+        `suffix` would otherwise land on the keyword argument of the same name.
+        """
         group_order = request.env["group.order"].sudo().browse(order_id).exists()
         url = self._eskaera_url(group_order, suffix=suffix)
         if post and url != "/eskaera":
@@ -729,7 +733,7 @@ class AplicoopWebsiteSale(WebsiteSale):
     @http.route(["/eskaera/<int:order_id>"], type="http", auth="user", website=True)
     def eskaera_shop_legacy(self, order_id, **post):
         """Keep the old numeric shop URL working, pointing at the slug one."""
-        return self._redirect_to_slug_url(order_id, **post)
+        return self._redirect_to_slug_url(order_id, post)
 
     @http.route(
         ["/eskaera/<string:group_order_slug>"],
@@ -1296,7 +1300,7 @@ class AplicoopWebsiteSale(WebsiteSale):
     )
     def eskaera_checkout_legacy(self, order_id, **post):
         """Keep the old numeric checkout URL working, pointing at the slug one."""
-        return self._redirect_to_slug_url(order_id, suffix="/checkout", **post)
+        return self._redirect_to_slug_url(order_id, post, suffix="/checkout")
 
     @http.route(
         ["/eskaera/<string:group_order_slug>/checkout"],