Fix clippy and formatting lints from stable toolchain drift

[subkanthi]

The CI workflow runs cargo clippy ... -- -D warnings and cargo fmt -- --check with a floating stable Rust toolchain. Newer stable releases (1.96+) introduced stricter lints that flag pre-existing code on the ClickHouse/v0.19.2-with-panic-safe-ffi base branch:

• dead_code: ColumnNamesAndTypes::extend in kernel/src/schema/mod.rs is never called. Fixed with #[allow(dead_code)].
• clippy::some_filter (new in 1.99): Some(x).filter(|_| false) in kernel/src/engine/parquet_row_group_skipping.rs — deliberately disabled code (references upstream issue Parquet row group skipping is broken/unsafe for missing columns delta-io/delta-kernel-rs#434). Fixed with #[allow(unknown_lints, clippy::some_filter)] to preserve intent across toolchain versions.
• unused_imports: IntoExternResult imported but unused in ffi/src/table_changes.rs. Removed.
• clippy::useless_conversion: .into_iter() on a Vec passed to .zip() in kernel/src/actions/visitors.rs test code. Simplified.
• cargo fmt: formatting drift in several files from the catch_unwind patches. Applied cargo fmt.

All lints are pre-existing on the base branch and unrelated to PR #1.

Failing CI run: https://github.com/Altinity/delta-kernel-rs/actions/runs/37404771416
Related: #1