Support retrieved_from: :doc_values for direct leaf fields

[jwils]

Summary

This PR adds a narrow secondary retrieval path for fields that should stay returnable in GraphQL while being excluded from stored _source.

t.field "internal_code", "String", retrieved_from: :doc_values

When a field is marked retrieved_from: :doc_values, ElasticGraph:

• keeps the field in GraphQL output types
• excludes the field from datastore _source via _source.excludes
• requests the field through datastore docvalue_fields
• resolves the field from fields in datastore hits when _source omits it

Implementation

The change is intentionally narrow:

• only direct fields on indexed root document types are supported
• only GraphQL leaf fields are supported
• dotted name_in_index paths are rejected
• text fields are rejected because they do not support datastore doc values
• query planning only uses docvalue_fields when every participating index definition agrees on the retrieval method

The runtime metadata now records how an index field is retrieved, the GraphQL datastore query can split requested fields between _source and docvalue_fields, and datastore response helpers can read direct leaf values back from hit fields when needed.

Tests

Updated unit specs cover:

• keeping retrieved_from: :doc_values fields in GraphQL output types
• emitting _source.excludes and runtime metadata for those fields
• requesting docvalue_fields and falling back to _source when index definitions disagree
• resolving direct scalar and list field values from hit fields
• rejecting unsupported retrieved_from usages

I was not able to run the RSpec files in this local environment because Bundler is missing required gems/native extensions.

Stack Info: Stacked on #1108

[myronmarston]

retrieved_from: :doc_values exposes a level of detail that, until now, we haven't exposed to ElasticGraph users. ElasticGraph does many things to query the data as efficiently as possible, without asking users to configure those things.

Can we do something similar here? For example, can we determine in which situations it's more efficient to use doc_values and automatically do so without requiring users to configure it at the field level?

Also, if we're going to change how fields are retrieved, I'd like to take a step back and review a full design that takes into account all the options Elasticsearch/OpenSearch offer us here. Besides _source and docvalue_fields there's also fields and stored_fields. I'd like to consider a unified design rather than just tacking this on as a one-off.