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 files or tests are named. Start by reproducing the queued-message workflow while a response is processing, then trace the Up-arrow edit and Enter submission paths in the CLI queue handling. Done means editing a queued message leaves only the edited version queued, with coverage for the reported reproduction steps.

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