```ts
/**
 * Parses a single line of CSV into its fields.
 * Handles quoted fields with escaped quotes inside.
 * Throws an Error if a quoted field is never closed.
 * @param line - The CSV line to parse.
 * @returns An array of fields.
 */
export function parseCsvLine(line: string): string[] {
  const fields: string[] = [];
  let currentField = '';
  let inQuotes = false;
  let escapeNext = false;

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

    if (escapeNext) {
      currentField += char;
      escapeNext = false;
    } else if (char === '"') {
      if (inQuotes && line[i + 1] === '"') {
        // Escaped quote
        currentField += '"';
        i++;
      } else {
        inQuotes = !inQuotes;
      }
    } else if (char === ',' && !inQuotes) {
      fields.push(currentField);
      currentField = '';
    } else {
      currentField += char;
    }
  }

  if (inQuotes) {
    throw new Error('Unclosed quoted field');
  }

  fields.push(currentField);

  return fields;
}
```