[BUG] validate index settings for models

[jn2clark]

Is your feature request related to a problem? Please describe.
the index settings validation does not catch the model field if it is in the incorrect part of the settings json

Describe the solution you'd like
if the model key is present in the settings but not nested correctly it should raise an error

Describe alternatives you've considered
none

Additional context
none

[jn2clark]

{
    "index_defaults": {
        "image_preprocessing": {
            "patch_method": null
        },
        "normalize_embeddings": true,
        "text_preprocessing": {
            "split_length": 2,
            "split_method": "sentence",
            "split_overlap": 0
        },
        "treat_urls_and_pointers_as_images": false
    },
    "model": "open_clip/ViT-L-14/laion2b_s32b_b82k",
    "number_of_shards": 5,
    "treat_urls_and_pointers_as_images": true
}

an example where the model key is incorrectly located but does not raise an error. this causes the default model to load