Added allow-expect-in-tests / allow-unwrap-in-tests (#2328)

This PR:
* Added the clippy.toml to configure allowable expect / unwrap usage in
tests
* Removed as many expect/allow lines as possible from tests
* moved a bunch of allows to expects where possible

Note: in integration tests, non `#[test]` helper functions are not
covered by this so we had to leave a few lingering `expect(expect_used`
checks around

codex-rs/exec/src/event_processor_with_json_output.rs

@@ -28,7 +28,7 @@ impl EventProcessor for EventProcessorWithJsonOutput {
             .into_iter()
             .map(|(key, value)| (key.to_string(), value))
             .collect::<HashMap<String, String>>();
-        #[allow(clippy::expect_used)]
+        #[expect(clippy::expect_used)]
         let config_json =
             serde_json::to_string(&entries).expect("Failed to serialize config summary to JSON");
         println!("{config_json}");