[codex] Use expect in integration tests (#28441)

The workspace denies `clippy::expect_used` in production. Although
`clippy.toml` allows `expect` in tests, Bazel Clippy compiles
integration-test helper code in a way that does not receive that
exemption, which encouraged verbose `unwrap_or_else(... panic!(...))`
and equivalent `match`/`let else` forms.

This allows `clippy::expect_used` once at each integration-test crate
root (including aggregated suites and test-support libraries), then
replaces manual panic-based Result and Option unwraps with
`expect`/`expect_err`. Standalone `tests/*.rs` files remain their own
crate roots. Intentional assertion and unexpected-variant panics remain
unchanged, and the production `expect_used = "deny"` lint remains in
place.

The cleanup is mechanical and net-negative in line count.

codex-rs/login/tests/all.rs

@@ -1,3 +1,5 @@
+#![allow(clippy::expect_used)]
+
 // Single integration test binary that aggregates all test modules.
 // The submodules live in `tests/suite/`.
 mod suite;