Context. After computing a report, ship it back as a CSV string (e.g., for download).
Your task. Write to_csv_string(df) that returns the DataFrame serialized as a CSV string with header, without the index column.
Example. A df with name/score → "name,score\nAlice,85\nBob,92\n".
Notes. Include header. Skip the index column (otherwise pandas prepends an unnamed column with row numbers). Trailing newline is part of pandas' default output.