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/Cargo.toml

@@ -26,6 +26,7 @@ edition = "2024"
 rust = { }
 
 [workspace.lints.clippy]
+expect_used = "deny"
 unwrap_used = "deny"
 
 [profile.release]