test(data): remove cache_dir deprecation warnings in cache tests

[novis10813]

Summary

• Migrate tests/data/test_cache.py and tests/data/test_cache_polars.py from deprecated BarCache(cache_dir=...) usage to BarCache(storage=..., cache_prefix="").
• Add shared fixtures using LocalStorageBackend so tests remain isolated by temporary directory while using the supported API.
• Keep test behavior unchanged (cache key/date-range/clear semantics), while eliminating deprecated constructor usage.

Verification

• uv run pytest tests/data/test_cache.py tests/data/test_cache_polars.py -W error::DeprecationWarning (14 passed)

Notes

• This PR intentionally touches only tests/data/* and does not include uv.lock or other stash contents.

[lilinoct18-coder]

it's a descent job, pretty good