BasicAuthenticator
Implements HTTP Basic authentication (RFC 7617).
Extracts BasicCredentials via BasicCredentialsResolver: missing credentials yield AuthenticationResult.NotAuthenticated; malformed credentials yield AuthenticationResult.Failure. Looks up the username via UserLookup and compares the raw password with PasswordEncoder; an unknown username or password mismatch yields AuthenticationResult.Failure. On success, returns AuthenticationResult.Success with the looked-up identity and roles. Construct via basicAuth { } or Builder.