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/project_doc.rs

@@ -134,7 +134,7 @@ async fn load_first_candidate(
 
 #[cfg(test)]
 mod tests {
-    #![allow(clippy::unwrap_used)]
+    #![allow(clippy::expect_used, clippy::unwrap_used)]
 
     use super::*;
     use crate::config::Config;