Empty assistant completions are rendered as "No response was returned" retry errors #5009

Describe the bug

When the model returns an empty assistant completion, Copilot CLI renders it as:

No response was returned. Send your message again to retry.

This is misleading: the conversation has advanced far enough for the next user message to be accepted, but the user sees what looks like a failed response. It also makes ordinary acknowledgements feel ignored or dismissive.

Observed repeatedly in one conversation after confirming that I had found information I first asked Terra to look up. The assistant produced an empty turn several times; each was displayed as the retry error above.

Affected version

Copilot CLI 1.0.89

Steps to reproduce the behavior

This appears intermittent/model-dependent rather than reliably prompt-reproducible:

1. Have an active Copilot CLI conversation with Terra.
2. Ask Terra to look up information.
3. After it responds, send a confirmation that you found the requested information.
4. If Terra emits an empty completion, observe the CLI transcript.

Expected behavior

Copilot CLI should not present an empty model completion as a request failure.

At minimum, it should distinguish an empty completion from a failed response and display a clear diagnostic. Ideally, the assistant should emit a brief acknowledgement instead of an empty visible turn.

Additional context

• OS: Windows
• Model during the observed empty responses: GPT-5.6 Terra (gpt-5.6-terra)
• The issue was observed multiple times on 2026-09-30.
• No model/provider error, content-filter message, or actionable failure detail was shown.