Virtual account creation accepts `provider`, `mode`, `markup`, `description`

What's new — POST /v1/virtual-accounts accepts four new request fields:

• provider — alias for bank using shorter labels (e.g. provider: "act" ↔ bank: "austin_capital_trust").
• mode — explicit account-mode selector (CRYPTO to auto-convert deposits, omit for fiat-balance mode).
• markup — per-VA fee/FX overrides.
• description — free-text label echoed back on reads.

Impact — Optional. Existing payloads that use bank and rely on default mode/fees continue to work unchanged.

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{
  "provider": "act",
  "mode": "CRYPTO",
  "markup": { "fx_bps": 25, "fee_bps": 10 },
  "description": "Acme Corp — primary settlement VA",
  "destination": { ... }
}

Reference — POST /v1/virtual-accounts