chore: improve slab maintenance path

[vip892766gma]

Summary:

• Add narrow edge-case unit tests covering currently-untested branches of the public API: e.g. (1) VacantEntry::key returning the same key that the subsequent insert assigns after compact() reshuffles indices, (2) Drain semantics when the slab is empty and when Drain is dropped before iteration completes (verifying remaining entries are cleared), and (3) get2_mut / get_disjoint_mut returning None for equal indices and for an out-of-bounds index paired with a valid one. Each test is a few lines, exercises an existing branch without changing behavior, and strengthens regression coverage.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.

[Darksonn]

Can you explain the name of this PR? I'm noticing other PRs with similar names, and the name does not make sense to me.

• chore: improve bytes maintenance path bytes#827
• chore: improve tower maintenance path tower-rs/tower#869
• chore: improve loom maintenance path loom#409