Support hovering over referenced models

We missed this in #343. When we hover over `test` in the `depends_on` array, we can see the YAML of the referenced service. We should support the same thing when hovering over `ai_model` in the `models` array.
```YAML
services:
  test:
    image: alpine
    models:
      - ai_model
  test2:
    depends_on:
      - test

models:
  ai_model:
    model: ai/model
```