Fix: Lower log level for closed-channel send (openai#9653)

## What?
- Downgrade the closed-channel send error log to debug in
`codex-rs/core/src/codex.rs`.

## Why?
- `async_channel::Sender::send` only fails when the channel is closed,
so the current error-level log is noisy during normal shutdown. See
issue openai#9652.

## How?
- Replace the error log with a debug log on send failure.

## Tests
- `just fmt`
- `just fix -p codex-core`
- `cargo test -p codex-core`

Author: Kbediako

codex-rs/core/src/codex.rs

@@ -1181,7 +1181,7 @@ impl Session {
         let rollout_items = vec![RolloutItem::EventMsg(event.msg.clone())];
         self.persist_rollout_items(&rollout_items).await;
         if let Err(e) = self.tx_event.send(event).await {
-            error!("failed to send tool call event: {e}");
+            debug!("dropping event because channel is closed: {e}");
         }
     }
 
@@ -1199,7 +1199,7 @@ impl Session {
             .await;
         self.flush_rollout().await;
         if let Err(e) = self.tx_event.send(event).await {
-            error!("failed to send tool call event: {e}");
+            debug!("dropping event because channel is closed: {e}");
         }
     }