Add forced shift assignment functionality for workers and update tests

rota/workers.py

@@ -118,6 +118,7 @@ class Worker(BaseModel):
     hard_day_exclusions: list[tuple[str, str]] = []  # e.g. [("Fri", "Sun")]
     force_assign_with: dict[str, list[str]] = {}  # e.g. {"oncall": ["weekend"]}
     max_shifts_per_week_by_shift_name: dict[str, int] = {}  # e.g. {"night": 2, "a": 3}
+    forced_assignments: List[tuple[int, str, str]] = []  # (week, day, shift_name)
 
 
     assign_as_block_preferences: dict[str, float] = {}  # shift_name -> weight (positive = prefer block)
@@ -430,4 +431,8 @@ class Worker(BaseModel):
         """Helper to set the maximum number of shifts of a given type per week for this worker."""
         if not hasattr(self, "max_shifts_per_week_by_shift_name"):
             self.max_shifts_per_week_by_shift_name = {}
-        self.max_shifts_per_week_by_shift_name[shift_name] = max_per_week
+        self.max_shifts_per_week_by_shift_name[shift_name] = max_per_week
+
+    def force_assign_shift(self, week: int, day: str, shift_name: str):
+        """Force this worker to be assigned to a shift on a specific week/day."""
+        self.forced_assignments.append((week, day, shift_name))