Refactor indexer lambda event decoding

[jwils]

Summary

• extract SQS event decoding behind a dedicated decoder object in elasticgraph-indexer_lambda
• keep JSON Lines as the default path while documenting the injected decoder contract on SqsProcessor
• add the missing JSONLDecoder / SqsProcessor RBS coverage and unit coverage for happy-path and empty-body decoding

Why

• prepares the runtime ingestion path for alternate payload formats without changing current JSONL behavior
• keeps the refactor narrowly scoped to the SQS/Lambda entrypoint on main

[myronmarston]

Looks good in general; left a few minor suggestions.

Can you explain how the protobuf ingestion support will build on this?

[myronmarston]

This is stateless, so we could define this as a module and do def self.decode_events below so that there's no garbage instance created for no reason.

[myronmarston]

What's the purpose of this line? It looks like it doesn't do anything...

More generally, how do you expect sqs_record to get used?

[myronmarston]

Suggested change

	body.split("\n").map { |event| JSON.parse(event) }
	body.split("\n").map { |event| ::JSON.parse(event) }

[myronmarston]

Suggested change

	def decode_events: (
	sqs_record: ::Hash[::String, untyped],
	body: ::String
	) -> ::Array[::Hash[::String, untyped]]
	extend _EventPayloadDecoder

This is the RBS equivalent of "implements the interface at the class level".

[myronmarston]

This should probably move out of SqsProcessor so it can be referenced without the SqsProcessor:: prefix. Maybe moved into the jsonl_decoder file?

[myronmarston]

Suggested change

	describe "#decode_events" do
	describe ".decode_events" do

(If you adopt my suggestion to make it callable directly on JSONLDecoder with no instance needed...).