channel: net: update unit-tests 4/N (#923)

Summary:
Pull Request resolved: #923

replace `tokio_util::codec::LengthDelimitedCodec`/`Framed` in the `MockLink` relay and remaining net.rs tests with `hyperactor::channel::net`’s `FrameReader`/`FrameWrite`. convert the relay to read via `FrameReader` and forward via `FrameWrite` over split `ReadHalf`/`WriteHalf`; drop `build_codec` and legacy `Framed`-based helpers; fold the `*2` helpers back into canonical names (`serve`, `take_receiver`, `verify_message`, `verify_stream`, `send_frame`, `write_stream`). behavior is unchanged; tests and the mock relay now exercise the production framer (zero-copy, cancellation-safe) directly.

Reviewed By: mariusae

Differential Revision: D80537409

fbshipit-source-id: ae3315c890c2113cce7494c131c6c4e222a4cda2

Author: shayne-fletcher