Move JSON schema helpers to json ingestion gem

[jwils]

Why

Keep the first extraction layer reviewable by moving the JSON Schema helper files without changing their contents, so GitHub can render them as renames instead of delete/add rewrites.

What

• Move JSON Schema helper Ruby and RBS files into elasticgraph-json_ingestion
• Update core requires/type references to load the helpers from the new gem
• Add the Docker/Apollo build-context wiring needed for the new schema_definition dependency

Risk Assessment

Low to medium - mostly file moves plus require/dependency wiring.

References

• Stacked on Add schema definition extension plumbing #1200.
• git diff -M --summary reports the moved helper files as 100% renames, including json_schema_with_metadata.rb.
• Local static checks passed for this layer before the Docker-only build-context amend; the full rewritten stack passes lint, type check, schema artifact check, dependency diagram verification, and spellcheck.