Disallow expect via lints (#865)

Adds `expect()` as a denied lint. Same deal applies with `unwrap()`
where we now need to put `#[expect(...` on ones that we legit want. Took
care to enable `expect()` in test contexts.

# Tests

```
cargo fmt
cargo clippy --all-features --all-targets --no-deps -- -D warnings
cargo test
```

codex-rs/mcp-server/src/codex_tool_runner.rs

@@ -19,6 +19,7 @@ use tokio::sync::mpsc::Sender;
 
 /// Convert a Codex [`Event`] to an MCP notification.
 fn codex_event_to_notification(event: &Event) -> JSONRPCMessage {
+    #[expect(clippy::expect_used)]
     JSONRPCMessage::Notification(mcp_types::JSONRPCNotification {
         jsonrpc: JSONRPC_VERSION.into(),
         method: "codex/event".into(),