chore: improve hyper-util maintenance path

[oab24413gmai]

Summary:

• Add focused unit tests covering edge cases of the Timer/TokioTimer wrappers (e.g., zero-duration sleep resolves immediately, reset() on an elapsed sleep, and Debug impl output), exercising existing public API without changing behavior.
• Keep the change narrow so it is straightforward to review.

Notes:

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