Add session header to chat widget (#3592)

<img width="570" height="332" alt="image"
src="https://github.com/user-attachments/assets/ca6dfcb0-f3a1-4b3e-978d-4f844ba77527"
/>

codex-rs/tui/src/chatwidget/session_header.rs

@@ -0,0 +1,16 @@
+pub(crate) struct SessionHeader {
+    model: String,
+}
+
+impl SessionHeader {
+    pub(crate) fn new(model: String) -> Self {
+        Self { model }
+    }
+
+    /// Updates the header's model text.
+    pub(crate) fn set_model(&mut self, model: &str) {
+        if self.model != model {
+            self.model = model.to_string();
+        }
+    }
+}