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/login/src/lib.rs

@@ -427,7 +427,6 @@ pub struct AuthDotJson {
 
 #[cfg(test)]
 mod tests {
-    #![expect(clippy::expect_used, clippy::unwrap_used)]
     use super::*;
     use crate::token_data::IdTokenInfo;
     use crate::token_data::KnownPlan;

codex-rs/login/src/token_data.rs

@@ -155,7 +155,6 @@ mod tests {
     use serde::Serialize;
 
     #[test]
-    #[expect(clippy::expect_used, clippy::unwrap_used)]
     fn id_token_info_parses_email_and_plan() {
         #[derive(Serialize)]
         struct Header {