Beneficiary schema corrected to match API behavior

Broker API

The Beneficiary schema on the account endpoints now matches what the API actually accepts and returns.

• middle_name is no longer required and accepts null. Omit it or send null when there is no middle name; an empty string is rejected. Account responses always include the field, using null when unset.
• relationship is limited to spouse and other.
• type is limited 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 limited to USA_SSN.
• tax_id requires a USA SSN on create and update requests; responses always return the masked value xxx-xx-xxxx.
• given_name, family_name, and middle_name reject empty strings.
• date_of_birth is documented as a YYYY-MM-DD date that cannot be in the future.
• share_pct is documented as a decimal string greater than 0 and at most 100, with at most two decimal places.

These values were already enforced by the API, so there is no new runtime rejection.

Affected Endpoints

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