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/tests/codex_message_processor_flow.rs

@@ -1,5 +1,3 @@
-#![allow(clippy::expect_used, clippy::unwrap_used)]
-
 use std::path::Path;
 
 use codex_core::spawn::CODEX_SANDBOX_NETWORK_DISABLED_ENV_VAR;