Avoid assigning new sessions if free disk space of agent is below a threshold

### Main idea

- [ ] Prevent creation of new sessions when insufficient storage is available by enabling the scheduler to check for minimum freespace.
- [ ] Display freespace status on agents when below a certain threshold.
- [ ] Include reasons for pending status in admin notifications, although they might be omitted if the scheduler excludes unprocessable items from the list.
### Alternative ideas

_No response_

### Anything else?

_No response_

JIRA Issue: BA-60