Function validateBankCode

validateBankCode(code: unknown): ValidationResult<BankInfo>
Distinguishes malformed four-digit input from an unsupported CBSL code.
Parameters
- code: unknown
  Four-digit CBSL bank code, as text or a number.
Returns ValidationResult<BankInfo>
{ isValid: true, data } with a copy of the bank, or a failure with code BANK_CODE_INVALID or BANK_CODE_UNSUPPORTED.
Example
```
validateBankCode('7010').isValid; // true
validateBankCode('0000').code; // 'BANK_CODE_UNSUPPORTED'
```
- Defined in bank/validator.ts:34