bug: Auto is not listed in /model when dynamicModelConfiguration is enabled

### What happened?

`Auto` is not listed in `model` when `dynamicModelConfiguration`(experimental) is enabled.

### What did you expect to happen?

Auto is listed in `/model`.

### Client information

Client Information

Run `gemini` to enter the interactive CLI, then run the `/about` command.

```console
▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄
> /about
▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀

╭──────────────────────────────────────────────────────────────────────╮
│ │
│ About Gemini CLI │
│ │
│ CLI Version 0.46.0-preview.1 │
│ Git Commit 0ded806d2 │
│ Model gemini-3.5-flash │
│ Sandbox no sandbox │
│ OS darwin │
│ Auth Method Signed in with Google (xxxxxxx@xxxxx.xxx) │
│ Tier Gemini Code Assist Standard │
│ GCP Project xxxxxxxxxxxxxxx │
│ │
╰──────────────────────────────────────────────────────────────────────╯
```

### Login information

Signed in with Google (Gemini Code Assist Standard)

### Anything else we need to know?

It can be workaround by
```json
"modelDefinitions": {
"auto": {
"isPreview": false
}
}
```