Get Global Node Config Schema

Endpoint to retrieve the JSON schema for GlobalNodeConfig.

Consumed by the dashboard's Super Node dialog to render the Routing & Guardrails sub-form via SchemaDrivenForm instead of hand-rolled MUI controls. Returning the Pydantic-derived schema keeps every leaf field (condition_freeform, condition_expression, args_schema, static_messages_config, dynamic_messages_config, reverse_conditional_edge, etc.) automatically in sync with the runtime config.