Use mem::take to drain unified exec output buffers (#39515)

## What changed

Replace the custom `HeadTailBuffer::drain` helper with `std::mem::take` at
the output collection call site. This moves the buffered output out while
resetting the shared buffer to its default empty state.

GitOrigin-RevId: 7a93d1312f2cea0d7913a5264eeb01726b45e0ea

codex-rs/core/src/unified_exec/head_tail_buffer.rs

@@ -93,16 +93,6 @@ impl<const MAX_BYTES: usize> HeadTailBuffer<MAX_BYTES> {
         out
     }
 
-    /// Drain the retained output and omission metadata, resetting this buffer's
-    /// contents while preserving its configured capacity.
-    pub(crate) fn drain(&mut self) -> Self {
-        Self {
-            head: std::mem::take(&mut self.head),
-            tail: std::mem::take(&mut self.tail),
-            omitted_bytes: std::mem::take(&mut self.omitted_bytes),
-        }
-    }
-
     /// Append a later buffer with the same budget. This preserves the summary
     /// of the original concatenated output, including its omission count.
     pub(crate) fn push_buffer(&mut self, mut buffer: Self) {

codex-rs/core/src/unified_exec/head_tail_buffer_tests.rs

@@ -42,24 +42,6 @@ fn head_budget_zero_keeps_only_last_byte_in_tail() {
     assert_eq!(buf.to_bytes(), b"c".to_vec());
 }
 
-#[test]
-fn draining_resets_state_and_push_buffer_preserves_omissions() {
-    let mut buf = HeadTailBuffer::<10>::default();
-    buf.push_chunk(b"0123456789".to_vec());
-    buf.push_chunk(b"ab".to_vec());
-
-    let drained = buf.drain();
-    let mut collected = HeadTailBuffer::<10>::default();
-    collected.push_buffer(drained);
-
-    assert_eq!(buf.retained_bytes(), 0);
-    assert_eq!(buf.omitted_bytes(), 0);
-    assert_eq!(buf.to_bytes(), b"".to_vec());
-    assert_eq!(collected.to_bytes(), b"01234789ab".to_vec());
-    assert_eq!(collected.omitted_bytes(), 2);
-    assert_eq!(collected.total_bytes(), 12);
-}
-
 #[test]
 fn chunk_larger_than_tail_budget_keeps_only_tail_end() {
     let mut buf = HeadTailBuffer::<10>::default();

codex-rs/core/src/unified_exec/process_manager.rs

@@ -1344,7 +1344,7 @@ impl UnifiedExecProcessManager {
             let mut wait_for_output = None;
             {
                 let mut guard = output_buffer.lock().await;
-                drained_output = guard.drain();
+                drained_output = std::mem::take(&mut *guard);
                 has_drained_output =
                     drained_output.retained_bytes() > 0 || drained_output.omitted_bytes() > 0;
                 if !has_drained_output {