fix(fusion_claims): default booking description + isolate order-less task test

Review follow-up: the base fusion.technician.task.description is required=True and
non-in-store tasks require an address (_check_address_required). So:
- action_book_from_wizard now defaults description to 'Service booking' when the
  payload carries neither description nor issue (avoids a required-field failure).
- test_task_without_order_is_allowed now sets description + is_in_store=True so it
  exercises only the relaxed _check_order_link, not those unrelated base constraints.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

fusion_claims/models/technician_task.py

@@ -533,7 +533,7 @@ class FusionTechnicianTaskClaims(models.Model):
             'duration_hours': dur,
             'is_in_store': in_shop,
             'x_fc_service_call_type': '%s_%s' % (category, timing),
-            'description': payload.get('description') or payload.get('issue') or '',
+            'description': payload.get('description') or payload.get('issue') or _('Service booking'),
         }
         if partner:
             task_vals['partner_id'] = partner.id