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/tests/sandbox.rs

@@ -1,6 +1,4 @@
 #![cfg(unix)]
-#![expect(clippy::expect_used)]
-
 use codex_core::protocol::SandboxPolicy;
 use codex_core::spawn::StdioPolicy;
 use std::collections::HashMap;
@@ -180,6 +178,7 @@ async fn allow_unix_socketpair_recvfrom() {
 
 const IN_SANDBOX_ENV_VAR: &str = "IN_SANDBOX";
 
+#[expect(clippy::expect_used)]
 pub async fn run_code_under_sandbox<F, Fut>(
     test_selector: &str,
     policy: &SandboxPolicy,