signMessage: add explicit input validation handling

The library previously rejected non-hex-string data but relied on an
underlying library to throw the error.

This makes the validation explicit, and changes the type of the `data`
parameter of `signMessage` from `string` to `Hex` to reflect that.

src/HDKeyring.test.ts

@@ -605,7 +605,7 @@ describe('hd-keyring', () => {
       const localMessage = 'hello there!';
       const msgHashHex = bufferToHex(
         Buffer.from(keccak256(Buffer.from(localMessage))),
-      );
+      ) as Hex;
       await keyring.addAccounts(9);
       const addresses = await keyring.getAccounts();
       const signatures = await Promise.all(
@@ -634,8 +634,8 @@ describe('hd-keyring', () => {
         numberOfAccounts: 1,
       });
 
-      await expect(keyring.signMessage(firstAcct, '')).rejects.toThrow(
-        'Cannot convert 0x to a BigInt',
+      await expect(keyring.signMessage(firstAcct, '' as Hex)).rejects.toThrow(
+        'Invalid input: data is not a hex string',
       );
     });
 

src/HDKeyring.ts

@@ -23,7 +23,8 @@ import {
   TypedDataV1,
   TypedMessage,
 } from '@metamask/eth-sig-util';
-import { Hex, Keyring, Eip1024EncryptedData } from '@metamask/utils';
+import type { Hex, Keyring, Eip1024EncryptedData } from '@metamask/utils';
+import { isStrictHexString } from '@metamask/utils';
 import { TxData, TypedTransaction } from '@ethereumjs/tx';
 import { HDKeyringErrors } from './errors';
 
@@ -261,9 +262,12 @@ export class HDKeyring implements Keyring<SerializedHdKeyringState> {
   // For eth_sign, we need to sign arbitrary data:
   async signMessage(
     address: Hex,
-    data: string,
+    data: Hex,
     opts: KeyringOpt = {},
   ): Promise<string> {
+    if (!isStrictHexString(data)) {
+      throw new Error('Invalid input: data is not a hex string');
+    }
     const message: string = stripHexPrefix(data);
     const privKey: Uint8Array = this.#getPrivateKeyFor(address, opts);
     const msgSig: ECDSASignature = ecsign(