Fix ScheduleRange validation

[hungrytech]

What was changed

The three-argument constructor accidentally checked step twice, so negative end values were accepted. end should follow the same non-negative constraint as start and step.

Why?

ScheduleRange(int start, int end, int step) checked step >= 0 twice and did not validate end.

As a result, negative end values were accepted even though schedule range values are expected to be non-negative. This change corrects the validation while preserving the existing constructor signatures and exception behavior.

Checklist

1. Closes ScheduleRange to check end #2728

2. How was this tested:

• Added ScheduleRangeTest covering negative end values in both constructors. The test fails on main and passes with this change.

[CLAassistant]

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