Remove @unchecked Sendable when NIOCore adopts Sendable

We added some `@unchecked Sendable` in #343 because NIOCore hasn't fully adopted `Sendable`. We can make these regular `Sendable` when that's done.

Start by reading issue #343 and verifying whether NIOCore has adopted Sendable. Then locate the @unchecked Sendable changes introduced there and determine which can become regular Sendable conformances. Done means the affected declarations no longer use the unchecked form and the repository’s tests pass.

Written by the indexing model from the issue text.