Up arrow to edit a queued message adds duplicate instead of replacing it

### Describe the bug

When the agent is actively processing a response and you have a message queued, pressing **Up arrow** recalls the queued message for editing. After editing and pressing Enter, the edited message is **added to the queue as a new entry** rather than **replacing** the original queued message. This results in both the original and the edited version being queued.

In Claude Code, the same workflow (Up arrow → edit queued message → Enter) correctly **replaces** the queued message with the edited version.

### Steps to reproduce the behavior

1. Send a prompt that triggers a long-running agent response
2. While the agent is working, type and submit a follow-up message (it gets queued)
3. Press Up arrow — the queued message appears in the input for editing
4. Edit the message and press Enter
5. **Actual:** Both the original and edited messages are now queued
6. **Expected:** The edited message replaces the original in the queue

### Expected behavior

Editing a queued message via Up arrow should replace the original queued message, not add a second one. This matches Claude Code behavior and is the intuitive expectation — the user is correcting what they queued, not adding another message.

### Additional context

Claude Code implements this as an edit/replace operation on the pending queue. Copilot CLI treats it as a new submission.

No se nombran archivos ni pruebas. Empieza reproduciendo el flujo de trabajo de los mensajes en cola mientras se procesa una respuesta y, después, sigue las rutas de edición con Up-arrow y de envío con Enter en el manejo de la cola de la CLI. Se considera completado cuando editar un mensaje en cola deja únicamente la versión editada en la cola, con cobertura de los pasos de reproducción indicados.

Escrito por el modelo de indexación a partir del texto del issue.