Generalize indexing schema versions

[jwils]

Why

The indexing pipeline spoke in json_schema_version. That key is specific to one ingestion format. A proto ingestion format has no JSON schemas, so it cannot supply that value.

This PR makes the shared pipeline speak in a format-neutral schema_version. Each ingestion format maps its own versioning concept onto that key.

What

• Add SCHEMA_VERSION_KEY = "schema_version" and use it in elasticgraph-indexer, elasticgraph-warehouse_lambda, and the shared test support.
• JSONIngestion::IngestionAdapter restores the json_schema_version key before it validates an event against the JSON schemas, so the artifacts stay unchanged. That patch is private to the JSON gem, so elasticgraph-schema_artifacts gains no new public API and keeps no knowledge of the generic key.
• JSONIngestion::IndexingEventDecoder maps the publisher's json_schema_version onto schema_version while it decodes JSON Lines.
• JSONIngestion::IngestionAdapter dispatches on schema_version and selects the closest available JSON schema version.

Publishers keep sending json_schema_version. The JSON schema artifacts keep the json_schema_version const. Only the decoded in-memory event uses the new key.

Deviation from the original PR

The original version of this PR also defaulted a missing schema version to the latest available version. I did not carry that over.

Ingestion adapter dispatch now uses handles_event?, which tests for the presence of schema_version. A default would make the JSON adapter silently accept any event without a version, so malformed events would fail late instead of early. A missing schema_version still produces a validation failure.

Verification

• elasticgraph-indexer, elasticgraph-json_ingestion, elasticgraph-schema_artifacts, and elasticgraph-warehouse_lambda unit suites: 566 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