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/mcp-server/src/outgoing_message.rs

@@ -101,7 +101,7 @@ impl OutgoingMessageSender {
         event: &Event,
         meta: Option<OutgoingNotificationMeta>,
     ) {
-        #[allow(clippy::expect_used)]
+        #[expect(clippy::expect_used)]
         let event_json = serde_json::to_value(event).expect("Event must serialize");
 
         let params = if let Ok(params) = serde_json::to_value(OutgoingNotificationParams {
@@ -244,8 +244,6 @@ pub(crate) struct OutgoingError {
 
 #[cfg(test)]
 mod tests {
-    #![allow(clippy::unwrap_used)]
-
     use codex_core::protocol::EventMsg;
     use codex_core::protocol::SessionConfiguredEvent;
     use pretty_assertions::assert_eq;