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/exec_linux.rs

@@ -27,8 +27,7 @@ pub fn exec_linux(
     let tool_call_output = std::thread::spawn(move || {
         let rt = tokio::runtime::Builder::new_current_thread()
             .enable_all()
-            .build()
-            .expect("Failed to create runtime");
+            .build()?;
 
         rt.block_on(async {
             let ExecParams {