fix(python): use non-deprecated params in search benchmark fixtures

[sezruby]

Problem

The dataset fixtures in python/benchmarks/test_search.py pass deprecated parameters to the dataset APIs:

• lance.write_dataset(..., use_legacy_format=False)
• lance.dataset(..., index_cache_size=64 * 1024)

The test config sets filterwarnings = ['error::DeprecationWarning', ...], so these emit DeprecationWarning as errors during fixture setup. As a result every benchmark in test_search.py errors out before running:

DeprecationWarning: use_legacy_format is deprecated, use data_storage_version instead
DeprecationWarning: The 'index_cache_size' parameter is deprecated. Use 'index_cache_size_bytes' instead.

Fix

Switch to the current parameters:

• use_legacy_format=False → data_storage_version="stable" (the exact mapping the deprecation shim applies).
• index_cache_size=64 * 1024 → index_cache_size_bytes=512 * 1024 * 1024 (512 MiB comfortably caches these 100k-row IVF_PQ indices).

Verification

$ uv run --group benchmarks pytest python/benchmarks/test_search.py::test_ann_no_refine --benchmark-only
test_ann_no_refine[clean]               541.48 us   1813 ops
test_ann_no_refine[with_delete_files]   770.46 us   1285 ops
test_ann_no_refine[with_new_rows]      2843.17 us    349 ops
3 passed

ruff check / ruff format --check clean.