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 is nullable. Omit it or send null when the beneficiary has no middle name; an empty string is rejected. Responses always include the field and return 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; responses always return the masked value xxx-xx-xxxx. The update example now sends a real SSN.
• date_of_birth uses date format (YYYY-MM-DD) and 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.
• given_name and family_name reject empty strings.

Affected Endpoints

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