Improve validation of prompt file headers

[aeschli]

Version: 1.101.0-insider (user setup)
Commit: 496ebc4
Date: 2025-05-21T05:04:29.987Z

The error message is a bit complicated and its unclear what is ignored.
I would think tools are ignore here and I would suggest the following

• squiggly only on tools
• error message

'ask' mode does not support tools. The listed tools are ignored.

---
mode: ask
tools: 

---

[aeschli]

Maybe best to say: Unknown tool name 'string'
Eventually we also want to support unquoted names in the array