Reject non-positive date_range steps

[CodingFeng101]

Summary

Reject date_range() steps that do not advance the generated date. Passing values such as months=0 or days=-1 currently allows the generator to yield the start date and then continue without making forward progress, which can hang callers that consume the full iterator with list(...).

Changes

• Add a positive-step validation in dateparser.date.date_range().
• Add a focused regression test for zero-month and negative-day steps.

Validation

• python -m pytest tests/test_date_range.py -q -> 2 passed
• Manual smoke check confirmed normal daily and monthly ranges still produce the same forward dates.