Invalid model in .prompt.md file blocks sending and subsequent messages

- Copilot Chat Extension Version: v0.61.0
- VS Code Version: 1.133.0 (Universal)
- OS Version: macOS Tahoe 26.5
- Feature (e.g. agent/edit/ask mode): All modes
- Selected model (e.g. GPT 4.1, Claude 3.7 Sonnet): n/a
- Logs: n/a

Steps to Reproduce:
1. Add an invalid model field to a .prompt.md frontmatter
e.g.
```md
---
name:
description:
agent:
model: 'does-not-exist'
---
```
2. Run the slash command in the chat input
```
/ ...
```
3. Try sending any followup message