Always request encrypted cot (#3539)

Otherwise future requests will fail with 500

codex-rs/core/src/client_common.rs

@@ -84,8 +84,10 @@ pub enum ResponseEvent {
 
 #[derive(Debug, Serialize)]
 pub(crate) struct Reasoning {
-    pub(crate) effort: ReasoningEffortConfig,
-    pub(crate) summary: ReasoningSummaryConfig,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub(crate) effort: Option<ReasoningEffortConfig>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub(crate) summary: Option<ReasoningSummaryConfig>,
 }
 
 /// Controls under the `text` field in the Responses API for GPT-5.
@@ -146,7 +148,10 @@ pub(crate) fn create_reasoning_param_for_request(
         return None;
     }
 
-    effort.map(|effort| Reasoning { effort, summary })
+    Some(Reasoning {
+        effort,
+        summary: Some(summary),
+    })
 }
 
 pub(crate) fn create_text_param_for_request(