[elezar]

Follow-up to #5219 (testkit cleanup hooks), addressing two design nits from its review.

• Replace the untyped Option<String> hook key with a crate-private Key enum. Variants are separate namespaces, so keys of different kinds can't collide. push_keyed takes a Key and remove takes &Key.
• Take the key out of Hook::new and add a keyed() builder. Anonymous user hooks have no key.

Pure refactor with no behavior change: ordering, dedupe, timeouts, and the public API are unchanged, and Key is crate-private. Only cleanup.rs and context.rs change.

Verified from tests/testkit: cargo fmt --all, cargo nextest run (12 pass), cargo clippy --all-targets -- -D warnings.

No linked issue: the author has write access on this repository, as AGENTS.md permits.