Unexpected chat message content type when running other tools in participant

[alexr00]

We don't limit the tools we use in our chat participant, which means that we could run tools that use prompt-tsx.

[rzhao271]

@alexr00 what are some verification steps for this issue?

[alexr00]

To verify:

1. Install pre-release of GHPR, VS Code Commander, and Copilot Chat
2. In the chat view: @githubpr Open the file explorer view
3. Verify that there are no errors (arguably, it should say "I can only help with GitHub stuff", but this bug was just about making sure there are no errors).