test(sdk-coin-polyx): cover missing wallet passphrase guard

Add a test asserting 'missing wallet passphrase' is thrown when
userKey and backupKey are present but walletPassphrase is not.

The automated review dispatched after the previous test-coverage
commit flagged that isMpcv2SigningMaterial()'s walletPassphrase
short-circuit and recover()'s outer 'missing wallet passphrase'
check were still only exercised indirectly, never via a real
(non-stubbed) call with that exact param combination.

Ticket: WCI-1228
Session-Id: 3a00bebe-fe12-4aea-9f1a-5a2d0dd8e94b
Task-Id: dfadd58f-a04c-4dd9-819b-506439735cec

Author: vibhavgo

modules/sdk-coin-polyx/test/unit/polyx.ts

@@ -293,6 +293,11 @@ describe('Polyx:', function () {
         const paramsWithoutBackupKey = { ...mpcV2RecoverParams, backupKey: undefined };
         await baseCoin.recover(paramsWithoutBackupKey).should.be.rejectedWith('missing backupKey');
       });
+
+      it('should throw missing wallet passphrase when userKey and backupKey are present but walletPassphrase is not', async function () {
+        const paramsWithoutPassphrase = { ...mpcV2RecoverParams, walletPassphrase: undefined };
+        await baseCoin.recover(paramsWithoutPassphrase).should.be.rejectedWith('missing wallet passphrase');
+      });
     });
   });