Enable allow_attributes lint for arrow-string

[Hashim1999164]

Which issue does this PR close?

Part of #10458.

Rationale for this change

Enable clippy::allow_attributes for arrow-string so future lint suppressions use expectations and stale suppressions are detectable. The existing large_enum_variant suppressions are still required on x86_64 Linux, so they are converted to target-specific expect attributes instead of unconditional allow attributes.

What changes are included in this PR?

• Deny clippy::allow_attributes in the arrow-string crate.
• Convert the two x86_64 Linux large_enum_variant suppressions to lint expectations.

Are there any user-facing changes?

No.

How was this change tested?

• cargo +1.96.1 fmt --all -- --check
• cargo +1.96.1 clippy -p arrow-string --all-targets --all-features -- -D warnings
• cargo +1.96.1 clippy -p arrow-string --target x86_64-unknown-linux-gnu --all-targets --all-features -- -D warnings
• cargo +1.96.1 test -p arrow-string --all-features (182 unit tests and 10 doc tests passed)

AI assistance

AI assistance was used to identify the lint occurrences and draft the minimal edits. I reviewed the full diff and validated it with formatting, Clippy, and the crate's tests.

[Hashim1999164]

The x86_64 Linux Clippy failure is fixed by using target-specific expect(clippy::large_enum_variant) attributes. I verified Clippy both locally and with --target x86_64-unknown-linux-gnu, plus 182 unit tests and 10 doc tests. The new external-contributor workflow runs are now action_required and need maintainer approval to start.