test(sdk-core): preserve allocation currency literal type

Keep the allocation currency inferred as a supported literal so the new
TradingNetwork test compiles in package builds.

WEB-000
PT-000000

Author: mmcshinsky-bitgo

modules/sdk-core/test/unit/bitgo/trading/network/network.test.ts

@@ -82,7 +82,7 @@ describe('TradingNetwork', function () {
   it('prepares a signed allocation with generated identifiers', async function () {
     const prepared = await tradingNetwork.prepareAllocation({
       connectionId: 'connection-id',
-      amount: { currency: 'tbtc', quantity: '100' },
+      amount: { currency: 'tbtc', quantity: '100' } as const,
       notes: 'test',
     });