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/protocol.rs

@@ -317,6 +317,11 @@ pub enum EventMsg {
         message: String,
     },
 
+    /// Reasoning event from agent.
+    AgentReasoning {
+        text: String,
+    },
+
     /// Ack the client's configure message.
     SessionConfigured {
         /// Tell the client what model is being queried.