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/is_safe_command.rs

@@ -75,6 +75,7 @@ fn is_safe_to_call_with_exec(command: &[String]) -> bool {
 fn try_parse_bash(bash_lc_arg: &str) -> Option<Tree> {
     let lang = BASH.into();
     let mut parser = Parser::new();
+    #[expect(clippy::expect_used)]
     parser.set_language(&lang).expect("load bash grammar");
 
     let old_tree: Option<&Tree> = None;