chore: improve async-std maintenance path

[oab24413gmai]

Summary:

• Add edge-case unit tests for BufRead/Read combinator behavior at EOF and empty-buffer boundaries (e.g. read_until with no delimiter, take(0) returning Ok(0) immediately), targeting branches currently lacking coverage.
• Keep the change narrow so it is straightforward to review.

Notes:

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