Namespace JSON schema helpers under JSON ingestion

[jwils]

Why

After the move-only PR preserves rename detection, put the moved helper constants under the ElasticGraph::JSONIngestion namespace.

What

• Namespace the moved JSON Schema helper classes and RBS signatures
• Update schema_definition references and tests to use the new qualified constants

Risk Assessment

Medium - namespace-only behavior should be unchanged, but it updates call sites across schema_definition.

References

• Stacked on Move JSON schema helpers to json ingestion gem #1201.
• Local lint, type check, schema artifact checks, and documentation coverage passed while building this split.

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