Fix histogram binwidth edge case

[ShiroKSH]

Fixes #3646.

Hist._define_bin_edges currently rounds a valid positive binwidth to zero bins whenever the data range is less than half the requested width. Both histplot and objects.Hist then pass zero to NumPy and raise a ValueError.

Keep the existing approximate-width behavior, but use one bin when a finite positive width would round to zero. For constant data, provide a one-width range so singleton histograms preserve the requested width, matching the earlier #2813 behavior. Invalid negative widths remain rejected.

Tests:

• uv run pytest -q tests/_stats/test_counting.py tests/test_distributions.py tests/test_objects.py (292 passed, 1 skipped)
• uv run ruff check seaborn/_stats/counting.py tests/_stats/test_counting.py
• uv run ty check

[ShiroKSH]

@mwaskom We'd appreciate it if you could take a look at this pull request when you have a chance.