Support returning default index_options for semantic_text fields when include_defaults is true

[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

[elasticsearchmachine]

Hi @kderusso, I've created a changelog YAML for you.

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[Mikep86]

Approach looks good overall, I have some follow-up questions about null serialization though.

[Mikep86]

Also, can we add a test showing that index_options provided by the user are always serialized?

[kderusso]

Also, can we add a test showing that index_options provided by the user are always serialized?

@Mikep86 Please see the tests starting here and let me know if you believe there's a gap.

[Mikep86]

@kderusso This test doesn't cover what I had in mind. I think it would be good to check that if we provide a mapping like:

PUT my-index
{
  "mappings": {
    "properties": {
      "dense_inference": {
        "type": "semantic_text",
        "inference_id": "some_dense_inference_id",
        "index_options": {
          <BBQ index options that match defaults>
        }
      }
    }
  }
}

The index_options are always serialized, even though they match defaults, because they are user-provided. This behavior differs from other field types, so good to capture in a test.

[Mikep86]

LGTM, thanks for the changes!