chore: improve hyper-util maintenance path

[oab24413gmai]

Summary:

• Add unit tests for the Rewind type covering pre-buffered read paths, partial reads spanning the rewound buffer and inner reader, and empty-prebuf behavior to lock in correctness of the AsyncRead implementation
• Keep the change narrow so it is straightforward to review.

Notes:

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