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_config.rs

@@ -117,6 +117,8 @@ pub(crate) fn create_tool_for_codex_tool_call_param() -> Tool {
         })
         .into_generator()
         .into_root_schema_for::<CodexToolCallParam>();
+
+    #[expect(clippy::expect_used)]
     let schema_value =
         serde_json::to_value(&schema).expect("Codex tool schema should serialise to JSON");
 
@@ -186,6 +188,7 @@ mod tests {
     #[test]
     fn verify_codex_tool_json_schema() {
         let tool = create_tool_for_codex_tool_call_param();
+        #[expect(clippy::expect_used)]
         let tool_json = serde_json::to_value(&tool).expect("tool serializes");
         let expected_tool_json = serde_json::json!({
           "name": "codex",