Exercise returnable: false widgets and preserve highlighting

[jwils]

Summary

• update the widgets test schema to exercise returnable: false on both a leaf field and an object field
• regenerate the affected schema artifacts and widget factory data for that schema change
• preserve highlighting for returnable: false fields by only excluding non-highlightable hidden fields from _source
• add unit coverage for the refined _source.excludes behavior
• add acceptance coverage proving hidden widget fields can still be filtered, sorted/grouped/aggregated, highlighted, and rejected from direct selection

Testing

• re-verified after reversing the stack: bundle exec rspec elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/datastore_config/index_mappings/miscellaneous_spec.rb
• in-branch coverage not rerun during this stack rewrite: NO_VCR=1 bundle exec rspec elasticgraph-graphql/spec/acceptance/returnable_fields_spec.rb
• in-branch coverage not rerun during this stack rewrite: script/lint elasticgraph-schema_definition/lib/elastic_graph/schema_definition/mixins/has_indices.rb elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/datastore_config/index_mappings/miscellaneous_spec.rb elasticgraph-graphql/spec/acceptance/returnable_fields_spec.rb

This supersedes merged PR #1120 after reversing the stack order.