Beneficiary schema corrected to match API behavior

Broker API

The Beneficiary schema now matches what the Accounts API actually accepts and returns.

• middle_name is no longer required and is nullable. Omit it or send null when the beneficiary has no middle name; an empty string is rejected. Account responses always include the field, using null when unset.
• relationship is restricted to spouse and other.
• type is restricted to primary and contingent. An account with beneficiaries needs at least one primary beneficiary, and share_pct must sum to 100 within the primary group and within the contingent group.
• tax_id_type is restricted to USA_SSN.
• tax_id is documented as requiring a USA SSN on create and update requests; account responses always return the masked value xxx-xx-xxxx.
• given_name, family_name, and middle_name require at least one character.
• date_of_birth uses date format (YYYY-MM-DD) and cannot be in the future.
• share_pct uses decimal format and is documented as greater than 0, at most 100, with at most two decimal places.

Affected Endpoints

• POST /v1/accounts
• GET /v1/accounts
• GET /v1/accounts/{account_id}
• PATCH /v1/accounts/{account_id}