fix(core): flaky test completed_commands_do_not_persist_sessions (#3596)

Fix flaky test:
```
        FAIL [   2.641s] codex-core unified_exec::tests::completed_commands_do_not_persist_sessions
  stdout ───

    running 1 test
    test unified_exec::tests::completed_commands_do_not_persist_sessions ... FAILED

    failures:

    failures:
        unified_exec::tests::completed_commands_do_not_persist_sessions

    test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 235 filtered out; finished in 2.63s
    
  stderr ───

    thread 'unified_exec::tests::completed_commands_do_not_persist_sessions' panicked at core/src/unified_exec/mod.rs:582:9:
    assertion failed: result.output.contains("codex")
```

codex-rs/core/src/exec_command/exec_command_session.rs

@@ -11,6 +11,9 @@ pub(crate) struct ExecCommandSession {
     /// Broadcast stream of output chunks read from the PTY. New subscribers
     /// receive only chunks emitted after they subscribe.
     output_tx: broadcast::Sender<Vec<u8>>,
+    /// Receiver subscribed before the child process starts emitting output so
+    /// the first caller can consume any early data without races.
+    initial_output_rx: StdMutex<Option<broadcast::Receiver<Vec<u8>>>>,
 
     /// Child killer handle for termination on drop (can signal independently
     /// of a thread blocked in `.wait()`).
@@ -42,6 +45,7 @@ impl ExecCommandSession {
         Self {
             writer_tx,
             output_tx,
+            initial_output_rx: StdMutex::new(None),
             killer: StdMutex::new(Some(killer)),
             reader_handle: StdMutex::new(Some(reader_handle)),
             writer_handle: StdMutex::new(Some(writer_handle)),
@@ -50,12 +54,26 @@ impl ExecCommandSession {
         }
     }
 
+    pub(crate) fn set_initial_output_receiver(&self, receiver: broadcast::Receiver<Vec<u8>>) {
+        if let Ok(mut guard) = self.initial_output_rx.lock()
+            && guard.is_none()
+        {
+            *guard = Some(receiver);
+        }
+    }
+
     pub(crate) fn writer_sender(&self) -> mpsc::Sender<Vec<u8>> {
         self.writer_tx.clone()
     }
 
     pub(crate) fn output_receiver(&self) -> broadcast::Receiver<Vec<u8>> {
-        self.output_tx.subscribe()
+        if let Ok(mut guard) = self.initial_output_rx.lock()
+            && let Some(receiver) = guard.take()
+        {
+            receiver
+        } else {
+            self.output_tx.subscribe()
+        }
     }
 
     pub(crate) fn has_exited(&self) -> bool {

codex-rs/core/src/exec_command/session_manager.rs

@@ -279,6 +279,7 @@ async fn create_exec_command_session(
     let (writer_tx, mut writer_rx) = mpsc::channel::<Vec<u8>>(128);
     // Broadcast for streaming PTY output to readers: subscribers receive from subscription time.
     let (output_tx, _) = tokio::sync::broadcast::channel::<Vec<u8>>(256);
+    let initial_output_rx = output_tx.subscribe();
 
     // Reader task: drain PTY and forward chunks to output channel.
     let mut reader = pair.master.try_clone_reader()?;
@@ -350,6 +351,7 @@ async fn create_exec_command_session(
         wait_handle,
         exit_status,
     );
+    session.set_initial_output_receiver(initial_output_rx);
     Ok((session, exit_rx))
 }