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 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 configurable through the JSON ingestion extension
• Update the generated JSON ingestion dependency diagram

Risk Assessment

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

References

• Stacked on Wire JSON ingestion schema extension modules #1215.
• Local script/quick_build passed before the generated diagram update; script/update_dependency_diagrams --verify and README spellcheck passed after it.

Stack

Current PR is marked with ->.

• #1199 Add JSON ingestion gem shell
• #1200 Add schema definition extension plumbing
• #1201 Move JSON schema helpers to json ingestion gem
• #1203 Namespace JSON schema helpers under JSON ingestion
• #1204 Add JSON ingestion indexing extensions
• #1215 Wire JSON ingestion schema extension modules
• -> #1205 Use JSON ingestion as schema definition extension
• #1202 Document JSON ingestion schema APIs
• #1224 Migrate JSON ingestion specs into gem
• #1220 Add configurable indexing event decoder