Subagent started with unsupported model fails the whole session

Describe the bug

In a long running agent session on autopilot (~2 hours), with plenty use of subagents, the main agent was Opus 5, but started gemini-3.6-flash when building and executing tasks. Later during that same autopilot session gemini-3.6-flash was no longer supported. The main agent attempts to launch gemini-3.6-flash agent again, which results in this error:

Execution failed: CAPIError: 400 The requested model is not supported.

This breaks the entire session and the main agent stops. Instead the main agent should have seen that error, been able to react on it and try to find a different model it could use (e.g. Haiku, GPT Luna), and launch the subagent

Affected version

GitHub Copilot CLI 1.0.79.

Steps to reproduce the behavior

1. Have a session running where you instruct the main agent to launch subagents using a model that you are going to retract from the pool of models
2. On your side, change which models are supported to ban a model that you made the session use
3. See the subagent fail and the main agent fail together

Expected behavior

The main agent should have seen that error, been able to react on it and try to find a different model it could use (e.g. Haiku, GPT Luna), and launch the subagent

Additional context

No response

Start by tracing how the CLI launches subagents and propagates model execution errors. No file or test is named, so locate the session and subagent error-handling entry points first; done means an unsupported subagent model failure no longer terminates the main session and can be handled for another model.