Add guard against division by zero in target_shifts calculation with warning logging

rota_generator/shifts.py

@@ -2007,6 +2007,19 @@ class RotaBuilder(object):
                         for i in shift.sites
                     )
 
+                    # Guard against division by zero: if no full-time
+                    # equivalent is available for the shift's sites the
+                    # denominator may be zero (for example when all site
+                    # FTEs are 0). In that case emit a warning and set the
+                    # target to 0 rather than raising an exception.
+                    if not full_time_equivalent_joined:
+                        # avoid ZeroDivisionError
+                        self.add_warning(
+                            "Zero FTE for sites",
+                            f"full_time_equivalent sum is zero for shift {shift.name}; setting target_shifts=0 for worker {worker.name}",
+                        )
+                        target_shifts = 0
+                    else:
                         target_shifts = (
                             total_shifts
                             / full_time_equivalent_joined