GET /me/available-cash and MCP get_available_cash

Account-level free cash across live credentials: GET /api/v1/me/available-cash sums free USDT/USDC/USD/FDUSD/BUSD per unique exchange (default credential), with by_exchange and incomplete when a venue fails. API keys need read:account. MCP get_available_cash and the public SDKs expose the same payload.