feat: reasoning effort as optional (#3527)

Allow the reasoning effort to be optional

codex-rs/protocol/src/mcp_protocol.rs

@@ -223,7 +223,8 @@ pub struct NewConversationResponse {
     pub conversation_id: ConversationId,
     pub model: String,
     /// Note this could be ignored by the model.
-    pub reasoning_effort: ReasoningEffort,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub reasoning_effort: Option<ReasoningEffort>,
     pub rollout_path: PathBuf,
 }
 
@@ -526,7 +527,8 @@ pub struct SendUserTurnParams {
     pub approval_policy: AskForApproval,
     pub sandbox_policy: SandboxPolicy,
     pub model: String,
-    pub effort: ReasoningEffort,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub effort: Option<ReasoningEffort>,
     pub summary: ReasoningSummary,
 }