chore: improve workers-rs maintenance path

[oab24413gmai]

Summary:

• Add or tighten focused edge-case tests or type assertions in test/src/d1.rs, test/src/kv.rs related to async networking, backend services, protocol correctness; avoid docs-only changes and broad refactors.
• Keep the change narrow so it is straightforward to review.

Notes:

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

[guybedford]

This seems auto-generated, and of little value, closing.