Agents call unsupported models for explore subagent

[liraz747]

Describe the bug

Whenever an agent in Copilot CLI tries to call the built-in explore agent, it hangs for a while then shows the error:

Error: Failed to get response from the
   AI model; retried 5 times (total
  retry wait time: 94.15796158848828
  seconds) (Request-ID
  ....)
  Last error: CAPIError: 400 The
  requested model is not supported.

Affected version

0.400

Steps to reproduce the behavior

1. Ask default agent to call explore subagent
2. Error occurs

Expected behavior

To not call unsupported models for the subagents

Additional context

• OS: Happens both on macOS and Linux
• Terminal Emulator: iTerm2 and Gnome Terminal
• Shell: bash and zsh