Use JSON ingestion as schema definition extension

[jwils]

Why

Complete the extraction by loading JSON ingestion through the schema-definition extension API and removing the duplicated core JSON Schema implementation.

What

• Wire ElasticGraph::JSONIngestion::SchemaDefinition::APIExtension explicitly into app, template, doctest, and spec-helper schema definition setup
• Move JSON Schema artifact/results behavior out of core schema_definition
• Remove JSON Schema field/type/index logic from core classes now provided by the extension
• Keep JSON schema version enforcement configured through schema.enforce_json_schema_version false

Risk Assessment

Medium to high — this is the behavior-transfer layer, but it sits after the move/namespace/add-extension PRs so the remaining diff is mostly deletion and wiring.

References

• Stacked on Add JSON ingestion schema extension modules #1204.
• Local lint, type check, spellcheck, targeted specs, schema artifact dump/check, and documentation coverage passed.
• Full local script/quick_build could not run because Docker and docker-compose are not installed, so no local datastore can be booted; GitHub CI should cover that environment.