Model selection is case sensitive

### Describe the bug

All the models are names with lower case. If a user inputs a upper case name, then the model picking will fail.

```
Model "GPT-5.4" is unsupported.

Available models:
- "claude-sonnet-4.6"
- "claude-sonnet-4.5"
- "claude-haiku-4.5"
- "claude-opus-4.6"
- "claude-opus-4.5"
- "claude-sonnet-4"
- "gemini-3-pro-preview"
- "gpt-5.4"
- "gpt-5.3-codex"
- "gpt-5.2-codex"
- "gpt-5.2"
- "gpt-5.1-codex-max"
- "gpt-5.1-codex"
- "gpt-5.1"
- "gpt-5.1-codex-mini"
- "gpt-5-mini"
- "gpt-4.1"

Supported models:
- "claude-opus-4.6-fast"
- "claude-opus-4.6-1m"

For information on Copilot policies and subscription, visit https://github.com/settings/copilot/features
```

### Affected version

GitHub Copilot CLI 1.0.2.

### Steps to reproduce the behavior

/models GPT-5.4

### Expected behavior

*the GPT-5.4 gets selected*

### Additional context

_No response_