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-types/tests/initialize.rs

@@ -1,3 +1,4 @@
+#![expect(clippy::expect_used)]
 use mcp_types::ClientCapabilities;
 use mcp_types::ClientRequest;
 use mcp_types::Implementation;

codex-rs/mcp-types/tests/progress_notification.rs

@@ -1,3 +1,4 @@
+#![expect(clippy::expect_used)]
 use mcp_types::JSONRPCMessage;
 use mcp_types::ProgressNotificationParams;
 use mcp_types::ProgressToken;