simulator: Fix timestamps manipulation

It's unclear whether the change in this mr triggers the bug in the simulator.
But it should be timestamp that needs manipulation and it's currently
datetimes. So this makes the current state fails with [1]

Adding the conversion layer from datetime to timestamps make the tests happier.

[1]
```
16:17:06  low = datetime.datetime(2025, 3, 14, 15, 17, 2, 139077, tzinfo=datetime.timezone.utc)
16:17:06  high = datetime.datetime(2025, 3, 14, 15, 17, 2, 139077, tzinfo=datetime.timezone.utc)
16:17:06
16:17:06      def _diff(low, high):
16:17:06          if low == high:
16:17:06              if low == 0:
16:17:06                  return 0.5
16:17:06              else:
16:17:06  >               return abs(low * 0.1)
16:17:06  E               TypeError: unsupported operand type(s) for *: 'datetime.datetime' and 'float'
16:17:06
```

Refs. swh/infra/sysadm-environment#5512

swh/scheduler/simulator/common.py

@@ -77,8 +77,24 @@ class SimulationReport:
             [runtime for runtime in runtimes if runtime <= self.DURATION_THRESHOLD]
         )
 
+    def _convert_timestamps(self, datetimes):
+        timestamps = []
+        convert_timestamps = False
+        for dt in datetimes:
+            if isinstance(dt, datetime):
+                convert_timestamps = True
+                timestamps.append(float(dt.timestamp()))
+
+        # If no conversion is needed, keep the initial timestamps
+        if not convert_timestamps:
+            timestamps = datetimes
+
+        return timestamps
+
     def metrics_plot(self) -> str:
-        timestamps, metric_lists = zip(*self.scheduler_metrics)
+        datetimes, metric_lists = zip(*self.scheduler_metrics)
+        timestamps = self._convert_timestamps(datetimes)
+
         known = [sum(m.origins_known for m in metrics) for metrics in metric_lists]
         never_visited = [
             sum(m.origins_never_visited for m in metrics) for metrics in metric_lists
@@ -87,10 +103,13 @@ class SimulationReport:
         figure = plotille.Figure()
         figure.x_label = "simulated time"
         figure.y_label = "origins"
+
         figure.scatter(timestamps, known, label="Known origins")
         figure.scatter(timestamps, never_visited, label="Origins never visited")
 
-        visit_timestamps, n_visits = zip(*self.visit_metrics)
+        visit_datetimes, n_visits = zip(*self.visit_metrics)
+        visit_timestamps = self._convert_timestamps(visit_datetimes)
+
         figure.scatter(visit_timestamps, n_visits, label="Visits over time")
 
         return figure.show(legend=True)

swh/scheduler/simulator/task_scheduler.py

@@ -22,8 +22,11 @@ def scheduler_runner_process(
     task_queues: Dict[str, Queue],
     min_batch_size: int,
 ) -> Iterator[Event]:
-    """Scheduler runner. Grabs next visits from the database according to the
-    scheduling policy, and fills the task_queues accordingly."""
+    """Simulate a scheduler runner process. This schedules tasks from the database
+    according to the scheduling policy, that means filling the task_queues first, then
+    update their status in the scheduler backend.
+
+    """
 
     while True:
         for visit_type, queue in task_queues.items():
@@ -49,6 +52,8 @@ def scheduler_runner_process(
             for sim_task in sim_tasks:
                 yield queue.put(sim_task)
 
+            env.timeout(0.1)
+
             # Then mark the sent tasks as scheduled
             env.scheduler.mass_schedule_task_runs(
                 [