feat: add as_series flag to maybe_get_index to return the index as a narwhals Series

[mvanhorn]

Description

Add a keyword-only as_series: bool = False argument to maybe_get_index. When as_series=True and the input is pandas-like, the index is returned wrapped as a narwhals Series (matching the input's implementation and stable-api version) instead of the raw native index. The default behavior is unchanged: as_series=False returns the raw native index, and non-pandas-like inputs return None.

This closes the request in #3693. maybe_get_index previously returned the raw pandas-like Index, which cannot be fed directly into maybe_set_index(..., index=...) because that parameter expects a narwhals Series (or list of them). Callers had to drop down to native objects and reassign .index by hand. With as_series=True, the result round-trips straight into maybe_set_index:

new_series = nw.maybe_set_index(new_series, index=nw.maybe_get_index(like, as_series=True))

Backend parity is preserved: the returned Series carries the same implementation as the input (pandas / modin / cuDF), and the stable v1 / v2 namespaces return their own Series type. Non-pandas-like inputs (e.g. Polars) still return None rather than an empty Series.

What type of PR is this? (check all applicable)

• 💾 Refactor
• ✨ Feature
• 🐛 Bug Fix
• 🔧 Optimization
• 📝 Documentation
• ✅ Test
• 🐳 Other

Related issues

• Closes Add argument in maybe_get_index to return the index as narwhals Series #3693

Checklist

• Code follows style guide (ruff)
• Tests added
• Documented the changes