Use JSON ingestion as schema definition extension

[jwils]

Why

Complete the extraction by loading JSON ingestion as the default schema_definition extension and removing the duplicated core JSON Schema implementation.

What

• Load elasticgraph-json_ingestion as the default schema definition extension when available
• 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
• Update tests, RBS signatures, and support wiring for the extracted default extension

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, schema artifact check, dependency diagram verification, and spellcheck passed.
• script/run_gem_specs elasticgraph-schema_definition could not run locally because no test datastore is running at http://localhost:9234; GitHub CI should cover that environment.