Corrected "kyc_results" schema on Broker account payloads

Broker API

Corrects the KYCResults schema to match the object shape the Broker API already returns. Runtime payloads are unchanged; this fixes the published contract that previously misrepresented the field.

• accept, indeterminate, and reject are now documented as nullable maps whose property names are KYC reason identifiers and whose values are always empty objects ({}).
• KYCResults is now nullable, representing responses that omit KYC outcomes with null.
• Documents 28 KYC reason identifiers in customer-facing language while keeping the set extensible, so integrations must tolerate and preserve unrecognized values.

Affected Endpoints

• GET /v1/accounts
• GET /v1/accounts/{account_id}
• PATCH /v1/accounts/{account_id}
• GET /v1/events/accounts/status