fix: address review feedback and improve Stellar tools validation

lib/stellar.ts

@@ -1,9 +1,36 @@
-import { Keypair, TransactionBuilder } from "stellar-sdk";
-
-export const signTransaction = (txXDR: string, networkPassphrase: string) => {
-    const keypair = Keypair.fromSecret(`${process.env.STELLAR_PRIVATE_KEY}`);
-    const transaction = TransactionBuilder.fromXDR(txXDR, networkPassphrase);
-    transaction.sign(keypair);
-    return transaction.toXDR();
-  };
-
+/// <reference types="node" />
+
+import { Keypair, TransactionBuilder } from "@stellar/stellar-sdk";
+
+/**
+ * Signs a Stellar transaction XDR string using the private key
+ * stored in environment variables.
+ */
+export const signTransaction = (
+  txXDR: string,
+  networkPassphrase: string
+): string => {
+  const secretKey = process.env.STELLAR_PRIVATE_KEY;
+
+  if (!secretKey) {
+    throw new Error("Missing STELLAR_PRIVATE_KEY in environment variables");
+  }
+
+  if (!txXDR) {
+    throw new Error("txXDR parameter is required");
+  }
+
+  if (!networkPassphrase) {
+    throw new Error("networkPassphrase parameter is required");
+  }
+
+  const keypair = Keypair.fromSecret(secretKey);
+  const transaction = TransactionBuilder.fromXDR(
+    txXDR,
+    networkPassphrase
+  );
+
+  transaction.sign(keypair);
+
+  return transaction.toXDR();
+};