Polish JSON ingestion schema API extensions

[jwils]

Why

Keep the JSON ingestion API polish separate from the README-only docs PR while preserving the stack order after JSON ingestion becomes a default schema-definition extension.

What

• Wrap JSON ingestion indexing field types instead of prepending behavior onto core field type classes
• Add JSON ingestion union type support and matching RBS signatures
• Update extension-module examples to append modules instead of replacing the default JSON ingestion extension
• Keep small wording fixes for JSON schema merge errors and metadata examples

Risk Assessment

Medium — this changes internal schema-definition extension wiring while preserving the generated JSON schema behavior.

References

• Stacked on Use JSON ingestion as schema definition extension #1205.
• Local script/lint, script/type_check, script/spellcheck, git diff --check, and affected schema-definition specs passed.