Party validations

Party data

• taxpayer.tax_registration_number: valid UAE TRN (15 digits, starts 1, ends 03).
• taxpayer.contact_phone: E.164 format.
• taxpayer.address.country_code: valid ISO 3166-1 alpha-2.
• taxpayer.address.country_subentity: valid UAE emirate code, only checked when country_code = AE.
• taxpayer.company_id_scheme_id: valid ISO ICD 6523 scheme identifier.
• taxpayer.company_id_scheme_agency_id: one of TL, CL, EID, PAS, CD.
• Cross-field: agency_id = TL → agency_name required (issuing authority name); agency_id = PAS → agency_name must be a valid ISO alpha-2 country code.
• Returns AlreadyExists if a party already exists for the branch.
• Update: all fields optional, evaluated against the merged state; tax_registration_number cannot change after creation (enforced implicitly - the field simply isn't on the update proto).