Expose the queued event count on CodexThread (#43918)

Add `CodexThread::queued_event_count()` to support a finite event drain
before transferring the receiver. Callers must remain the sole event reader
until they consume the snapshot count, leaving later events for the next reader.

GitOrigin-RevId: 55f9dd1cf8ebd396fdb60a1dc3d1428ed28cc2d2

codex-rs/core/src/codex_thread.rs

@@ -587,6 +587,14 @@ impl CodexThread {
         self.io.next_event().await
     }
 
+    /// Returns the event count for a finite drain before transferring the receiver.
+    ///
+    /// The caller must own the only event reader until it consumes this many events.
+    /// Events queued after this snapshot remain for the next reader.
+    pub fn queued_event_count(&self) -> usize {
+        self.io.rx_event.len()
+    }
+
     pub async fn agent_status(&self) -> AgentStatus {
         self.io.agent_status().await
     }