ci(python): typecheck arrow.py and test_arrow.py

[YuangGao]

Summary

Add lance/arrow.py and tests/test_arrow.py to the pyright include list and fix the resulting type errors. Closes #3292.

• arrow.py: narrow union types via typed locals / typing.cast, annotate the internal decoder return types, switch the fallback to pyarrow.compute as pc, and add a single # pyright: ignore[reportOptionalCall] for tf.stack (typed as None by tensorflow's incomplete stubs).
• lance/lance/init.pyi: fix the bfloat16_array stub signature from List[str | None] to Sequence[float | None] — the Rust function takes Vec<Option<f32>>, and Sequence (covariant) lets list[float] callers type-check.
• test_arrow.py: add import lance.arrow so the lance.arrow.* attribute accesses resolve.

Test plan

tensorflow is linux-only in pyproject.toml, so it was installed locally to mirror the CI (Linux) lint environment before checking.

• pyright → 0 errors (15 warnings, all pre-existing missing-stub warnings for pyarrow/pandas/etc.)
• ruff format --check + ruff check → pass
• pytest python/tests/test_arrow.py → 20 passed

[wjones127]

Looks nicely written. There are a few failures, but once those are addressed I'll be able to approve.

[YuangGao]

@wjones127 Thanks! pushed a fix