```ts
/**
 * Encodes a string using run-length encoding.
 * Replaces every maximal run of one repeated character with that character
 * followed by the run length. A run of length one is encoded with its count.
 * @param input The string to encode.
 * @returns The run-length encoded string.
 * @throws Error if input is not a string.
 */
export function runLengthEncode(input: string): string {
  if (typeof input !== 'string') {
    throw new Error('Input must be a string');
  }

  if (input.length === 0) {
    return '';
  }

  let result = '';
  let currentChar = input[0];
  let count = 1;

  for (let i = 1; i <= input.length; i++) {
    if (i < input.length && input[i] === currentChar) {
      count++;
    } else {
      result += currentChar + count;
      if (i < input.length) {
        currentChar = input[i];
        count = 1;
      }
    }
  }

  return result;
}
```