Wire JSON ingestion schema extension

[jwils]

Why

After the JSON ingestion gem exists, schema definition needs a concrete extension implementation so JSON Schema artifacts are generated through opt-in extension modules instead of core schema-definition code.

Stacked on #1221.

What

• Add JSON-ingestion indexing wrappers for fields, field references, and field types.
• Add schema definition extension modules for API, factory, results, state, schema artifact management, and schema elements.
• Wire generated projects and built-in gems to use JSON ingestion as a schema definition extension.
• Remove JSON Schema behavior from core schema definition classes where the extension now owns it.

Verification

• script/run_gem_specs elasticgraph-json_ingestion (skips because this extracted gem does not have its own spec directory yet)
• bundle exec rspec elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/json_schema_spec.rb elasticgraph-schema_definition/spec/integration/elastic_graph/schema_definition/rake_tasks_spec.rb elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/factory_spec.rb elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/indexing/json_schema_with_metadata_spec.rb elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/json_schema_field_metadata_spec.rb elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/json_schema_pruner_spec.rb elasticgraph-indexer/spec/unit/elastic_graph/indexer/record_preparer_spec.rb elasticgraph-apollo/spec/unit/elastic_graph/apollo/apollo_directives_spec.rb elasticgraph-apollo/spec/unit/elastic_graph/apollo/schema_definition_spec.rb
• script/type_check
• script/quick_build passes spellcheck, lint, type check, and schema artifact checks locally; it stops before the full spec run because the local Docker/test datastore is not running at http://localhost:9234.