feat: include reasoning_effort in NewConversationResponse (#3506)

`ClientRequest::NewConversation` picks up the reasoning level from the user's defaults in `config.toml`, so it should be reported in `NewConversationResponse`.

codex-rs/protocol/src/mcp_protocol.rs

@@ -217,6 +217,8 @@ pub struct NewConversationParams {
 pub struct NewConversationResponse {
     pub conversation_id: ConversationId,
     pub model: String,
+    /// Note this could be ignored by the model.
+    pub reasoning_effort: ReasoningEffort,
     pub rollout_path: PathBuf,
 }