Adding GPT 4o to Github Copilot CLI

### Describe the feature or problem you'd like to solve

GPT 4o is a wonderful model that is present in Copilot Chat, and in the Github Copilot extension for VS Code but is currently missing in Github CLI.

### Proposed solution

Add GPT 4o access to Copilot CLI

### Example prompts or workflows

_No response_

### Additional context

_No response_

The issue names no files or tests. Start by locating the Copilot CLI's model-access path and how available models are registered; done means GPT-4o can be selected and used through the CLI, with a test or documented verification path.

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