Allow custom model ID input for Gemini with custom base URL

[lingdongs]

When configuring the Google Gemini provider with a "Custom Base URL," the model selection is limited to a hardcoded dropdown list. This prevents users from specifying custom model IDs that are available through their proxy or self-hosted services (e.g., one-api, new-api, or other custom endpoints).

The ability to use a custom base URL is fantastic, but its utility is greatly diminished if we cannot also specify custom model names that the target endpoint serves. The current implementation assumes the custom URL will serve the exact same model names as Google's official API, which is often not the case.