Add configurable indexing event decoder

[jwils]

Why

The indexer lambda currently assumes SQS payload bodies are JSON Lines. To support ingestion sources that serialize events differently, the payload decoding step needs to be configurable before the normal ElasticGraph event validation and indexing pipeline runs.

What

• Add an indexer.indexing_event_decoder extension setting using the existing runtime extension loader pattern.
• Add a default ElasticGraph::Indexer::IndexingEventDecoder::JSONLines decoder to preserve current behavior.
• Wire elasticgraph-indexer_lambda SQS processing through the configured decoder while keeping warehouse lambda compatibility.
• Add RBS, focused specs, README docs, and regenerated config schema.

Verification

• script/type_check
• bundle exec rspec elasticgraph-indexer/spec/unit/elastic_graph/indexer/config_spec.rb elasticgraph-indexer/spec/unit/elastic_graph/indexer_spec.rb elasticgraph-indexer_lambda/spec/unit/elastic_graph/indexer_lambda/sqs_processor_spec.rb elasticgraph-indexer_lambda/spec/unit/elastic_graph/indexer_lambda/lambda_function_spec.rb elasticgraph-warehouse_lambda/spec/unit/elastic_graph/warehouse_lambda/lambda_function_spec.rb
• bundle exec standardrb elasticgraph-indexer/lib/elastic_graph/indexer.rb elasticgraph-indexer/lib/elastic_graph/indexer/config.rb elasticgraph-indexer/lib/elastic_graph/indexer/event_id.rb elasticgraph-indexer/lib/elastic_graph/indexer/indexing_event_decoder.rb elasticgraph-indexer/spec/unit/elastic_graph/indexer/config_spec.rb elasticgraph-indexer/spec/unit/elastic_graph/indexer_spec.rb elasticgraph-indexer/spec/support/example_extensions/indexing_event_decoder.rb elasticgraph-indexer_lambda/lib/elastic_graph/indexer_lambda/lambda_function.rb elasticgraph-indexer_lambda/lib/elastic_graph/indexer_lambda/sqs_processor.rb elasticgraph-indexer_lambda/spec/unit/elastic_graph/indexer_lambda/sqs_processor_spec.rb
• script/update_config_artifacts --verify

Design notes

• SqsProcessor takes the decoder as a required keyword argument; both the indexer lambda and the warehouse lambda pass their indexer's configured decoder, so the JSON Lines default is defined in exactly one place (the indexer.indexing_event_decoder config default) and decoders can rely on receiving non-nil schema_artifacts.
• The default JSONLines decoder lives in elasticgraph-indexer (rather than elasticgraph-json_ingestion) because the indexer needs a default it can require without depending on an optional extension gem. If/when JSON-specific indexer logic moves into elasticgraph-json_ingestion (per the long-term direction discussed on Add JSON ingestion gem shell #1199), the decoder can move with it.

Stack

Current PR is marked with ->.

• #1199 Add JSON ingestion gem shell
• #1200 Add schema definition extension plumbing
• #1201 Move JSON schema helpers to json ingestion gem
• #1203 Namespace JSON schema helpers under JSON ingestion
• #1204 Add JSON ingestion indexing extensions
• #1215 Wire JSON ingestion schema extension modules
• #1205 Use JSON ingestion as schema definition extension
• #1202 Document JSON ingestion schema APIs
• #1224 Migrate JSON ingestion specs into gem
• -> #1220 Add configurable indexing event decoder

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