Provision a managed Umbra wallet (Mode A)

Create a Solrouter-custodied Umbra wallet. The keypair is generated server-side and envelope-encrypted (AES-256-GCM) — the agent never holds private keys. Fund the returned `umbraAddress`, then run swaps and encrypted-balance operations against the wallet id.

curl -X POST "https://example.com/agents/v1/wallets" \  -H "Content-Type: application/json" \  -d '{    "webhookUrl": "https://my-agent.example/hooks/umbra"  }'

{
  "walletId": "f47ac10b-58cc-4372-a567-0e02b2c3d479",
  "umbraAddress": "umbra1q9c0ffee7p3rh4z8x2v6m5k0n9d3w7s2a1b4c8e6f0g",
  "network": "mainnet",
  "fundingHint": "send SOL or SPL tokens to umbraAddress to fund this wallet"
}

{
  "error": "missing_required_fields"
}