Identity schema corrected: decimal strings and nullable "tax_id_type"

Broker API

Corrects the Identity schema to match what the API actually accepts and returns.

• annual_income_min, annual_income_max, liquid_net_worth_min, liquid_net_worth_max, total_net_worth_min, and total_net_worth_max are now documented as decimal-formatted strings instead of numbers, matching the quoted values returned in responses. Examples have been added for each.
• tax_id_type is no longer unconditionally required and may be null on read. It is required on write only when tax_id is supplied; some account types return identity data without it.

Affected Endpoints

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