Fix invalid JSON Schema in listing_config_schema.json

[Manya Sharma (ManyaS-Git)]

Summary

.ci/listing_config_schema.json declares $schema: https://json-schema.org/draft/2020-12/schema, but it was not valid against that meta-schema. Validating with the jsonschema library surfaced 21 meta-schema violations, from two distinct problems:

1. "type": "list" is not a valid JSON Schema type (offer_listing)

The listing_contacts, listing_uris, listing_logos, listing_screenshots and listing_videos fields used "type": "list". list is not a valid JSON Schema type — these are collections, so the correct type is "array".

2. plan_overview was structurally malformed

• items.properties contained a nested {"type": "array", "items": {...}} wrapper that held the actual plan object, so the property names type/items were treated as schema properties and the real plan properties were never applied.
• A duplicate copy of the plan schema was also placed at the array level under a properties keyword, which is meaningless on an array schema and was silently ignored.

The real plan properties (plan_listing, pricing_and_availability, technical_configuration) are now declared directly under items.properties, and the duplicated array-level block is removed.

Verification (with the jsonschema library, draft 2020-12)

Check	Before	After
Valid against declared meta-schema	INVALID (21 violations)	VALID (0 violations)
Accepts a realistic configuration instance	–	✓
Rejects malformed plan data (e.g. plan_listing: 123)	silently accepted	✓ rejected

This means the schema now actually constrains the plan data it was meant to describe instead of accepting anything.