Serialize default index_options for semantic_text fields

[kderusso]

This PR fixes an issue from #119967 where the default index_options were not displayed when include_defaults is set to true.

Part of this fix requires serializing the model_settings earlier so they're accessible when we're performing serialization checks for index_options.

Here is an example of how to test this:

PUT my-dense-semantic-index/
{
  "mappings": {
    "properties": {
      "inference_field": {
        "type": "semantic_text",
        "inference_id": "my-e5-model"
        }
      }
    }
  }
}

// Should not return index_options
GET my-dense-semantic-index/_mapping

// Returns BBQ hnsw default index_options
GET my-dense-semantic-index/_mapping/field/inference_field?include_defaults

[kderusso]

Closing as this introduces inconsistency in serialized models.