setSessionToken method
Set session token for Authorization: Bearer …. Replaces the legacy
app-key header that v2 used. The session token is a per-checkout
credential minted by the merchant backend via
POST /api/v1/internal/sessions/create on the gateway.
Implementation
void setSessionToken(String sessionToken) {
_dio.options.headers['Authorization'] = 'Bearer $sessionToken';
// Defense in depth: drop the legacy header if anything ever sets it.
_dio.options.headers.remove('app-key');
}