Add step to Cube rolling windows

[Kkkakania]

Description

Adds a step argument to Cube.rolling_window, matching the strided-window support already provided by iris.util.rolling_window. The default remains step=1, so existing calls keep their current behavior.

The step is applied consistently to cube shape selection, data windows, and numeric/string coordinates. Invalid non-positive steps now raise the same clear error as the utility function.

Fixes #7217.

Checklist

• Included a What's New entry
• Incorporated type hints in changed code
• Added tests for data, dataless cubes, coordinates, and validation
• Checked that no benchmark update is needed
• Updated the public method docstring
• Checked that no dependency update is needed

Validation

• pytest lib/iris/tests/unit/cube/test_Cube.py -q (496 passed, 34 skipped)
• pytest lib/iris/tests/test_analysis.py::TestRollingWindow -q (9 passed)
• pre-commit run --files lib/iris/cube.py lib/iris/tests/unit/cube/test_Cube.py changelog/7232.feature.rst

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[trexfeathers]

Sorry @Kkkakania, this clashes with the fix proposed by the original requester (#7236), so I'm unable to merge it. Although judging by your contributions graph, and the pending CLA signature, I'm guessing you don't need us to bolster your GitHub CV.

Thanks for getting in touch!

[Kkkakania]

Understood, thanks for letting me know. Closing this in favor of #7236 makes sense.