Bug: 'text.response' in custom theme triggers validation error (unrecognized key)

### Description
The `text.response` key in a custom theme configuration triggers a validation error: `Invalid configuration... Unrecognized key(s) in object: 'response'`.

### Expected Behavior
The `response` key should be recognized as a valid property of the `text` object in a custom theme, as it is documented in `cli_help` and explicitly used in the source code to color model responses.

### Actual Behavior
The CLI flags `response` as an unrecognized key during startup validation, although the color is correctly applied in the terminal.

### Evidence
In the bundled source code (`chunk-7DZN7VCC.js`), the CLI explicitly looks for this key:
```javascript
response: customTheme.text?.response ?? customTheme.text?.primary ?? colors.Foreground
```

However, the `CustomTheme` definition in the validation schema (and the hosted `settings.schema.json`) is missing the `response` property under `text` and has `additionalProperties: false` set:

```json
"text": {
"type": "object",
"additionalProperties": false,
"properties": {
"primary": { "type": "string" },
"secondary": { "type": "string" },
"link": { "type": "string" },
"accent": { "type": "string" }
}
}
```

### Environment
- Gemini CLI Version: 0.38.2
- OS: Darwin (macOS)