Add per-task total_increasing completion counter sensor

[Copilot]

This adds a new sensor per task that tracks how many times the task has been completed, enabling trend/statistics use-cases. The counter increments only on actual “mark as done” actions (button/service), and does not change when setting last_done directly.

• Completion counter model

  • Added times_completed to TaskTrackerCoordinator.
  • Increment happens in async_mark_as_done(...) after a successful state transition path.
  • async_set_last_done_date(...) remains non-incrementing.

• New sensor entity

  • Added TaskTrackerTimesCompletedSensor for each task device.
  • Sensor uses state_class: total_increasing.
  • Stable entity naming/unique ID pattern:
    • sensor.task_tracker_<task_name>_times_completed
    • <entry_id>_times_completed
  • Startup restore logic keeps the highest known counter value to avoid regressions on reload.

• Coverage updates

  • Added focused integration assertions for:
    • sensor creation + state_class
    • increment via task_tracker.mark_as_done
    • increment via task button press
    • no increment via task_tracker.set_last_done_date

# coordinator.py
async def async_mark_as_done(self, today: date) -> None:
    ...
    self.times_completed += 1
    self._async_notify_listeners()

• Fixes Feature Request: a 'number of times completed' sensor #68