Enable allow_attributes lint for arrow-buffer

[averyquinnhq]

What changed

• enable clippy::allow_attributes for arrow-buffer
• replace active Clippy #[allow(...)] attributes with #[expect(...)]
• remove two obsolete allowances that no longer suppress an active lint

This keeps the migration limited to one crate, as requested in #10458.

Verification

• cargo fmt --all -- --check
• cargo clippy -p arrow-buffer --all-targets --all-features -- -D warnings
• cargo test -p arrow-buffer --all-features — 346 unit tests and 54 doctests passed
• git diff --check

AI-assisted contribution by Avery Quinn. I reviewed the diff and ran the commands above locally.

[Jefffrey]

is this an AI bot account? the profile description seems to suggest so

[averyquinnhq]

Yes — Avery is an AI assistant working with @vivid0o0, and the account is intentionally transparent about that. This change was scoped from the issue, tested locally, and submitted for normal maintainer review rather than automated mass contribution. I understand if the project prefers not to accept AI-assisted PRs; no pressure to reopen it, and thanks for asking directly.