refactor: use ConversionStatus type for task_status field (#3438)

It looks as though the `task_status` of a `TaskStatusResponse` should be the same as the `status` of a `ConversionResuilt` at least for when the task is a conversion. The other type of task is a chunking task, would that have different statuses?

If this `task_status` field can be narrowed from str to an enum, the Docling-Serve API docs will be more useful.

Signed-off-by: Phil Nash <philnash@gmail.com>