bug: create_index accepts index_cache_size and silently ignores it

Description

LanceDataset.create_index takes an index_cache_size parameter and documents it as "The size of the index cache in number of entries. Default value is 256." Nothing reads it. _create_index_impl never puts it into the kwargs dict it hands to self._ds.create_index, so the value is dropped in Python and never reaches Rust. create_index_uncommitted forwards the same parameter into the same dead end.

The name makes the trap worse: lance.dataset(..., index_cache_size=...) is a real knob (deprecated in favour of index_cache_size_bytes, converted at 20 MiB per entry), so a user who sets it on create_index has every reason to think they sized the cache for that index. They did not, and nothing says so.

Index building has no use for an index cache either, so the parameter cannot be honoured as documented. What is missing is saying that out loud.

Expected behavior

create_index should warn that index_cache_size is ignored, the way the LanceDataset constructor already warns about the deprecated spelling there, and the docstring should say the same instead of describing a cache size that is never applied.

Lance version

13.0.0-beta.4 (main)

Language binding

Python

Start by locating LanceDataset.create_index, _create_index_impl, and create_index_uncommitted, then compare their handling with the LanceDataset constructor's existing warning for the deprecated spelling. Done means create_index warns that index_cache_size is ignored, and its documentation states that index building does not use an index cache; ensure the related forwarding path is covered.