```ts
/**
 * Encodes a string by replacing every maximal run of one repeated character with that character followed by the run length.
 * @param input The string to encode.
 * @returns The encoded string.
 */
export function runLengthEncode(input: string): string {
  if (typeof input !== 'string') {
    throw new Error('Input must be a string');
  }

  let result = '';
  let count = 1;

  for (let i = 0; i < input.length; i++) {
    const char = input[i];
    const nextChar = input[i + 1];

    if (char === nextChar) {
      count++;
    } else {
      result += char;
      if (count > 1) {
        result += count.toString();
      }
      count = 1;
    }
  }

  return result;
}
```