feat: include "reasoning" messages in Rust TUI (#892)

As shown in the screenshot, we now include reasoning messages from the
model in the TUI under the heading "codex reasoning":


![image](https://github.com/user-attachments/assets/d8eb3dc3-2f9f-4e95-847e-d24b421249a8)

To ensure these are visible by default when using `o4-mini`, this also
changes the default value for `summary` (formerly `generate_summary`,
which is deprecated in favor of `summary` according to the docs) from
unset to `"auto"`.

codex-rs/core/src/models.rs

@@ -33,6 +33,10 @@ pub enum ResponseItem {
         role: String,
         content: Vec<ContentItem>,
     },
+    Reasoning {
+        id: String,
+        summary: Vec<ReasoningItemReasoningSummary>,
+    },
     FunctionCall {
         name: String,
         // The Responses API returns the function call arguments as a *string* that contains
@@ -67,6 +71,12 @@ impl From<ResponseInputItem> for ResponseItem {
     }
 }
 
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(tag = "type", rename_all = "snake_case")]
+pub enum ReasoningItemReasoningSummary {
+    SummaryText { text: String },
+}
+
 impl From<Vec<InputItem>> for ResponseInputItem {
     fn from(items: Vec<InputItem>) -> Self {
         Self::Message {