Release builds emit an unused_mut warning in app-server

What issue are you seeing?

Building codex-app-server in release mode emits an unused_mut warning:

warning: variable does not need to be mutable
    --> app-server/src/lib.rs:1297:5
     |
1297 |     mut loader_overrides: LoaderOverrides,
     |     ----^^^^^^^^^^^^^^^^
     |     |
     |     help: remove this `mut`

What steps can reproduce the bug?

1. Clone the repository.
2. Enter the Rust workspace:

cd codex-rs

1. Build app-server in release mode:

cargo build --release -p codex-app-server

What is the expected behavior?

Release builds should complete without an unused_mut warning, while debug
builds should continue allowing loader_overrides.user_config_path to be
modified.
A minimal possible fix is to allow this lint only when debug assertions are
disabled:

#[cfg_attr(not(debug_assertions), allow(unused_mut))]
fn loader_overrides_with_test_user_config_file(
    mut loader_overrides: LoaderOverrides,
    test_user_config_file: Option<std::path::PathBuf>,
) -> IoResult<LoaderOverrides> {
    // ...
}

Additional information

I have tested this change locally and can submit a focused one-line pull request
if a maintainer considers this approach appropriate and invites the
contribution.
Proposed fix commit:
https://github.com/GhostBoyBoy/codex/commit/30267fd50e