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/core/tests/prompt_caching.rs

@@ -1,5 +1,3 @@
-#![allow(clippy::expect_used, clippy::unwrap_used)]
-
 use codex_core::ConversationManager;
 use codex_core::ModelProviderInfo;
 use codex_core::built_in_model_providers;
@@ -24,7 +22,6 @@ fn sse_completed(id: &str) -> String {
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn prefixes_context_and_instructions_once_and_consistently_across_requests() {
-    #![allow(clippy::unwrap_used)]
     use pretty_assertions::assert_eq;
 
     let server = MockServer::start().await;