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/codex.rs

@@ -1654,6 +1654,7 @@ fn format_exec_output(output: &str, exit_code: i32, duration: std::time::Duratio
         },
     };
 
+    #[expect(clippy::expect_used)]
     serde_json::to_string(&payload).expect("serialize ExecOutput")
 }