CompanyListing

Inline schema type for the curated listings[] array on CompanySerializer.

These 6 fields are what CompanySerializer.get_listings returns — a
de-duplicated, equity-only subset of SecurityListing rows. Using an
explicit serializer here (rather than SecurityListingSerializer) keeps
the OpenAPI contract tight and avoids advertising the 11-field full model
shape that includes company, market_sector, etc.

{
    "mic": "string",
    "ticker": "string",
    "exchange_name": "string",
    "exch_code": "string",
    "security_type": "string",
    "figi": "string"
}