Add multiple BYOK model capability in copilot cli

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

Currently copilot CLI supports a single BYOK model with env var. I want to enable multiple models through it which is difficult. Inside copilot TUI, I can NOT switch to another BYOK model, I have to terminate the session and set the new env vars and then reopen copilot CLI again.

### Proposed solution

Provide a json in copilot directory which copilot cli will read and use to list all the models inside.

### Example prompts or workflows

I could then do this

copilot --model

instead of resetting the env variables

### Additional context

_No response_

Start by tracing how the Copilot CLI currently reads the single BYOK model from environment variables and how model selection works in the TUI. Define how the proposed JSON configuration and `--model` option should expose multiple models, with completion meaning users can switch models without terminating and reopening the CLI.

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