Pending tasks are not cleared

[callumforrester]

Tasks are currently only removed from pending_tasks when clear_task is called on it, meaning pending_tasks contain tasks that are not is_pending, and various states of is_complete/is_error.
I think we need pending_tasks, active_task, finished_tasks eventually.

[callumforrester]

Apologies for past-me's unclear rambling, but I think this means we should rename worker.pending_tasks to simply worker.tasks, since not all of them are, in fact, pending (some are already done, for example)

[stan-dot]

`
class TrackableTask(BlueapiBaseModel, Generic[T]):
"""
A representation of a task that the worker recognizes
"""

task_id: str
task: T
is_complete: bool = False
is_pending: bool = True
errors: List[str] = Field(default_factory=list)`

what could also change - the name of the TrackableTask or the property that is accessed with `.task`. that Writing task.task is odd. Maybe taskdata? or just 'data'?

[stan-dot]

adding tests to pass coverage check