Two issues fixed:
1. Alipay.SupportedTypes() returned ["alipay_direct"] and Wxpay returned
["wxpay_direct"], but the frontend sends payment_type="alipay"/"wxpay".
The registry lookup failed with "payment method (alipay) is not
configured". Fix: return the base types ["alipay"]/["wxpay"].
2. When multiple providers support the same payment type (e.g. EasyPay
and Alipay direct both handle "alipay"), only the last-registered
provider's instances were reachable — the registry mapped one type to
one provider key, and SelectInstance queried by that single key.
Fix: bypass the registry in invokeProvider and let SelectInstance
query across all providers when providerKey is empty. The selected
instance's own ProviderKey (now included in InstanceSelection) is
used to create the correct provider, enabling true cross-provider
load balancing.
Closes#1592
f498eb8fde