[Feature] is_valid: for job_shop_lib/_schedule.py

[drQuesadaUPM]

Description

Propose adding a new function is_valid in job_shop_lib/_schedule.py to validate schedule instances.

Expected Behavior

The function is_valid(schedule) should:

1. Return True if all constraints are satisfied.
2. Return False or raise an exception with a clear message if any constraint is violated.

Affected Files

• job_shop_lib/_schedule.py

References

• Potential constraints to check:
  • No overlapping tasks on the same machine.
  • All start and end times are positive.
  • Each job follows the defined operation order.