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.

[seanmonstar]

It looks like this was posted 4 times, so I assume we can close #287, #288, and #289.

[sjcobb2022]

@seanmonstar this is definitely either just spam or someone has pointed a openclaw instance here or something.