fix: preserve RangeIndex.arange step

[jaythehardcoder]

Description

RangeIndex.arange currently computes the transform size from (stop - start) / step, but leaves RangeCoordinateTransform.step to be derived from (stop - start) / size. When step does not evenly divide the interval, that changes the materialized coordinate values.

For example, RangeIndex.arange(0.0, 1.0, 0.3, dim="x") currently produces coordinates spaced by 0.25 instead of matching np.arange(0.0, 1.0, 0.3).

This preserves the requested step on RangeIndex.arange transforms and propagates the logical spacing through slice-derived transforms.

Checklist

• Closes RangeIndex.arange does not preserve step, has different values than numpy.arange #11325
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
    Tools: Hermes Agent / ChatGPT

Test Plan

• uv run pytest xarray/tests/test_range_index.py -q
• uv run ruff check xarray/indexes/range_index.py xarray/tests/test_range_index.py

Closes #11325

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[cfriedland5]

Assigning a different value than would be computed to a property's cache seems brittle