feat(sdk-coin-starknet): add isSignablePreHashed to starknet

CECHO-1376

TICKET: CECHO-1376

Author: shubham-damkondwar

modules/sdk-coin-starknet/src/starknet.ts

@@ -15,6 +15,7 @@ import {
   VerifyTransactionOptions,
   verifyMPCWalletAddress,
   UnexpectedAddressError,
+  SignableTransaction,
 } from '@bitgo/sdk-core';
 import { coins, BaseCoin as StaticsBaseCoin } from '@bitgo/statics';
 import { createHash, Hash } from 'crypto';
@@ -126,6 +127,14 @@ export class Starknet extends BaseCoin {
     return {};
   }
 
+  /**
+   * Returns true because Starknet's signableHex is already the final 32-byte hash.
+   * This prevents ecdsaMPCv2 from hashing the payload a second time.
+   */
+  isSignablePreHashed(unsignedTx: SignableTransaction): boolean {
+    return true;
+  }
+
   public generateKeyPair(seed?: Buffer): KeyPair {
     return utils.generateKeyPair(seed);
   }

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

@@ -87,6 +87,10 @@ describe('Starknet', function () {
     it('should return ECDSA as MPC algorithm', function () {
       basecoin.getMPCAlgorithm().should.equal('ecdsa');
     });
+
+    it('should identify as pre-hashed signable', function () {
+      basecoin.isSignablePreHashed({} as any).should.equal(true);
+    });
   });
 
   describe('Key pair generation', () => {