Fix the monthly active users count

An agent wrote a 40-line SQL query for monthly active users, a number that goes to the board. The numbers look plausible, but nobody on the team can follow the joins. Before trusting it, you compute the same metric a second, simpler way, straight from the raw events, and compare.

An assistant wrote distinct_active_users(event_rows) as that second count. It is in the editor. It has a bug. Find it and fix it.

Input and output:

• event_rows: one event per row, written user_id|date, with the date as YYYY-MM-DD.
• Return one YYYY-MM=count string per month that has an active user, sorted by month.

Rules the tests check:

• Split each row at its first |. Skip a row with no | or with an empty user id.
• The month is the first 7 characters of the date: 4 digits, -, then 2 digits from 01 to 12. Skip a row whose date does not start that way. The rest of the date is not checked.
• A month's count is the number of different users with at least one event in it. A user counts once per month, however many rows they have in that month.
• User ids compare exactly: case and spaces matter.