feat(wasm-solana): add serializeMessage() for web3.js API compatibility

Add serializeMessage() method to Transaction class that returns the
signable payload as a Buffer. This provides compatibility with code
expecting the @solana/web3.js Transaction.serializeMessage() API,
which returns "the Transaction data that need to be covered by signatures".

Ticket: BTC-2955

Author: lcovar

packages/wasm-solana/bitgo-wasm-solana-0.0.1.tgz

@@ -88,6 +88,16 @@ export class Transaction {
     return this._wasm.signable_payload();
   }
 
+  /**
+   * Serialize the message portion of the transaction.
+   * Alias for signablePayload() - provides compatibility with @solana/web3.js API.
+   * Returns a Buffer for compatibility with code expecting .toString('base64').
+   * @returns The serialized message bytes as a Buffer
+   */
+  serializeMessage(): Buffer {
+    return Buffer.from(this.signablePayload());
+  }
+
   /**
    * Serialize the transaction to bytes
    * @returns The serialized transaction bytes