Unify Copilot model settings to dropdown like `chat.utilityModel`

[mamaruo]

These settings currently accept a manually entered model ID, which makes them hard to discover and easy to misconfigure:

• github.copilot.selectedCompletionModel
• github.copilot.chat.askAgent.model
• github.copilot.chat.exploreAgent.model
• github.copilot.chat.implementAgent.model

Request:

• Change these settings from free-form text input to a dropdown selector, like the existing chat.utilityModel setting.

Question:

Is there a supported way to list all available model IDs programmatically or from configuration? If not, could VS Code or Copilot expose one?