test(amber): add unit test coverage for Schedule iterator semantics

[aglinxinyuan]

Task Summary

Schedule (amber/src/main/scala/org/apache/texera/amber/engine/architecture/scheduling/Schedule.scala) is central to region-by-region workflow execution but currently has no dedicated unit tests. Add ScheduleSpec covering its iterator semantics:

• getRegions flattens all level sets
• Iteration follows ascending level keys starting from the minimum
• hasNext / next() semantics: empty schedule, exhaustion, non-zero start
• Gap-stop behavior on non-contiguous level keys

Priority

P3 – Low

Task Type

• Testing / QA