Select Copilot model and default to `gpt-4o`

[dustinbyrne]

This PR adds a new command AppMap: Select Copilot Model that allows users to change their preferred Copilot model. The default model is set to gpt-4o.

Changes

• Added a new command appmap.copilot.selectModel that lets users pick from available Copilot models
• Added configuration setting appMap.copilot.preferredModel to store the user's model preference
• Set default model to gpt-4o
• The RPC server restarts after model selection to apply changes
• Added unit tests to verify the model selection functionality

Testing

The changes have been tested to verify:

• Model selection UI works correctly
• Setting is saved properly
• RPC server restarts after model change
• Error handling for when no models are available
• Cancellation handling for the model picker

Notes

The command can be accessed via:

• Command palette: AppMap: Select Copilot Model
• Default model is gpt-4o if no preference is set

[kgilpin]

Hi @dustinbyrne thanks! I have taken a look, I think @dividedmind should take a look too, just to be sure we don't miss anything.