Add automatic 0x prefix to private key handling (#229)

* feat: add auto-prefix 0x to private key handling

Previously, the code would throw an error if the WALLET_PRIVATE_KEY didn't start with '0x'. 
This update improves the user experience by automatically adding the '0x' prefix if missing, 
while maintaining the validation for the correct key length.

The code will work seamlessly whether users provide the private key with or without 
the '0x' prefix, making it more flexible and user-friendly.

* Remove rebundant error

---------

Co-authored-by: Agustin Armellini Fischer <[email protected]>

typescript/examples/vercel-ai/mode/index.ts

@@ -17,7 +17,16 @@ import { viem } from "@goat-sdk/wallet-viem";
 
 require("dotenv").config();
 
-const account = privateKeyToAccount(process.env.WALLET_PRIVATE_KEY as `0x${string}`);
+let privateKey = process.env.WALLET_PRIVATE_KEY || "";
+
+if (!privateKey) {
+    throw new Error("WALLET_PRIVATE_KEY environment variable is required");
+}
+
+// Normalize private key format
+privateKey = privateKey.startsWith("0x") ? privateKey : `0x${privateKey}`;
+
+const account = privateKeyToAccount(privateKey as `0x${string}`);
 
 const walletClient = createWalletClient({
     account: account,