utilAesDecrypt and utilAesEncrypt key parameter should be optional in type definition

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The key parameter in utilAesDecrypt and utilAesEncrypt has a default value at runtime (DEFAULT_128), but the TypeScript type definition marks it as required.

In src/aes.ts:

export function utilAesDecrypt(cipherText: string, key: number[]): string {
  key = key || DEFAULT_128;  // runtime default exists
  ...
}

The type signature should be:

export function utilAesDecrypt(cipherText: string, key?: number[]): string;
export function utilAesEncrypt(plainText: string, key?: number[]): string;

This would allow calling the function without providing a key, matching the runtime behavior.