Disallow expect via lints (#865)

Adds `expect()` as a denied lint. Same deal applies with `unwrap()`
where we now need to put `#[expect(...` on ones that we legit want. Took
care to enable `expect()` in test contexts.

# Tests

```
cargo fmt
cargo clippy --all-features --all-targets --no-deps -- -D warnings
cargo test
```

codex-rs/core/src/landlock.rs

@@ -140,7 +140,7 @@ fn install_network_seccomp_filter_on_current_thread() -> std::result::Result<(),
 
 #[cfg(test)]
 mod tests {
-    #![allow(clippy::unwrap_used)]
+    #![expect(clippy::unwrap_used, clippy::expect_used)]
 
     use super::*;
     use crate::exec::ExecParams;