This PR switches the OpenRouter integration from fetching all models to
fetching only the models specified in the user's account preferences.
This will help improve the experience

**The Problem**

The previous implementation used the `/models` endpoint, which returned
an exhaustive list of all models supported by OpenRouter. This resulted
in a long and cluttered model selection dropdown in Zed, making it
difficult for users to find the models they actually use.

**The Solution**

We now use the `/models/user` endpoint. This API call returns a curated
list based on the models and providers the user has selected in their
[OpenRouter dashboard](https://openrouter.ai/models).

Ref: [OpenRouter API Docs for User-Filtered
Models](https://openrouter.ai/docs/api-reference/list-models-filtered-by-user-provider-preferences)

Release Notes:

- language_models: Support OpenRouter user preferences for available
models