Parse an HTTP Authorization: Basic <base64> header into credentials.
solve('Basic dXNlcjpwYXNz') → { username: 'user', password: 'pass' }
Use `Buffer.from(b64, 'base64').toString('utf8')` (Node.js built-in).
Rules:
null for missing/invalid prefix or missing colon.: separates username from password; the password may contain colons.Sample tests