Moves `use codex_core::protocol::EventMsg` inside the block annotated
with `#[cfg(debug_assertions)]` since that was the only place in the
file that was using it.
This eliminates the `warning: unused import:` when building with `cargo
build --release` in `cargo-rs/tui`.
Note this was not breaking CI because we do not build release builds on
CI since we're impatient :P