Don't serialize endpoint metadata in semantic text mappings

[Mikep86]

In #141393, MinimalServiceSettings was updated to add endpoint metadata. This change introduced a bug (see #144969) where endpoint metadata is serialized in semantic_text index mappings. Endpoint metadata is only necessary when selecting the default inference service to use; once the inference service is selected it is no longer useful and can be discarded.

This PR fixes that by implementing filtered XContent for MinimalServiceSettings that excludes endpoint metadata. An alternative approach considered was to make a new class, MappingServiceSettings, that models all params in MinimalServiceSettings except for endpoint metadata. We did not choose to go with that for a couple reasons:

• Due to this bug, there are some users out there with semantic_text index mappings on disk with endpoint metadata. We need to know how to parse those or the mappings will be invalid, breaking Elasticsearch. See testMappingWithEndpointMetadata for a test validating this case with the chosen approach.
• There are places where we rely on the semantic_text model settings being MinimalServiceSettings, such as when updating the inference ID. Using a new class like MappingServiceSettings breaks this logic, and would require more significant changes to resolve.

Nothing precludes us from refactoring at a later time to introduce a new class like MappingServiceSettings. This PR aims to fix the bug with the fewest and simplest changes possible so that it can be resolved quickly and without side-effects.

Fixes #144969

[elasticsearchmachine]

Pinging @elastic/es-search-foundations (Team:Search Foundations)

[elasticsearchmachine]

Hi @Mikep86, I've created a changelog YAML for you.

[jonathan-buttner]

Ideally in the future we can remove MinimalServiceSettings from implementing ServiceSettings. Maybe when we get around to doing that we can rename the method then. The other option we have is to leverage the Params instead which I think is the typical way to handle this.

[jonathan-buttner]

If we wanted to use Params here instead, I think we'd do something like this:

(b, n, v) -> {
  if (v != null) {
    b.field(MODEL_SETTINGS_FIELD, v, new ToXContent.MapParams(Map.of("exclude_metadata", "true")));
  }
}

[Mikep86]

@elasticmachine update branch