payment_methods` field on Virtual Account responses

GET /virtual-accounts and GET /virtual-accounts/:id now return a payment_methods object describing the enabled inbound and outbound payment rails for each virtual account.

{
  "id": "...",
  // ...existing fields...
  "payment_methods": {
    "inbound": [
      { "name": "WIRE", "status": "active" },
      { "name": "ACH", "status": "coming-soon" }
    ],
    "outbound": [
      { "name": "WIRE", "status": "active" },
      { "name": "SWIFT", "status": "disabled" }
    ]
  }
}

- Each entry contains a name (payment rail identifier) and a status (active, coming-soon, or disabled).
- payment_methods is null when the provider does not expose payment method configuration.
- inbound or outbound may individually be null if only one direction is configured.