Extract indexing field metadata behind a format-neutral value object

[jwils]

Why

Indexer::RecordPreparer read its per-field indexing metadata straight out of the JSON schema artifacts. It received raw hashes shaped like {"type" => "ID!", "nameInIndex" => "id"} and called eg_meta.fetch("nameInIndex") directly.

That made the JSON schema representation part of the indexer's record-preparation contract. elasticgraph-indexer is meant to stay ingestion-format-neutral, but an ingestion format with no JSON schemas (such as the planned elasticgraph-proto_ingestion runtime) could only supply metadata by synthesizing JSON-schema-shaped hashes with camelCase keys.

What

• Add RecordPreparer::FieldMetadata, a value object with type and name_in_index.
• RecordPreparer::TypeMetadata now exposes fields_by_name (a Hash<String, FieldMetadata>) instead of eg_meta_by_field_name.
• RecordPreparer uses field.name_in_index and field.type instead of fetching camelCase JSON schema keys.
• JSONIngestion::RecordPreparerFactory converts the JSON schema ElasticGraph metadata into FieldMetadata, so the JSON schema key names stay inside elasticgraph-json_ingestion.
• Add a spec that builds a RecordPreparer directly from TypeMetadata and FieldMetadata, with no JSON schema involved. This is the path a non-JSON ingestion format will use.

Behavior is unchanged. This is a pure refactor of the contract between the indexer and the ingestion format gems.

Follow-up

This removes the shape coupling. A later PR still needs to give non-JSON formats a source for this metadata. Today name_in_index and the indexing type are recorded only in the versioned JSON schema artifacts.

Runtime metadata is not a sufficient substitute. Its graphql_fields_by_name entries record name_in_index, but they carry no field type, which RecordPreparer needs to select a scalar indexing preparer and to recurse into nested objects. The entries are also unversioned and describe the GraphQL field set, not the ingestion field set.

Verification

• elasticgraph-indexer, elasticgraph-json_ingestion, elasticgraph-indexer_lambda, elasticgraph-warehouse_lambda, elasticgraph-lambda_support, elasticgraph-warehouse, elasticgraph-local and elasticgraph-apollo unit suites: 673 examples, 0 failures
• script/lint, script/spellcheck, script/type_check: all green
• bundle exec rake schema_artifacts:check: artifacts up to date

Stack

Current PR is marked with ->.

• #1301 Extract an ingestion adapter seam inside elasticgraph-indexer
• #1302 Move JSON ingestion into elasticgraph-json_ingestion via an indexer extension
• #1220 Add configurable indexing event decoder
• -> #1351 Extract indexing field metadata behind a format-neutral value object
• #1284 Generalize indexing schema versions