Fix NoSQL Injection

The function below builds a MongoDB query filter by embedding user input directly into a $where clause string.

The bug: $where runs its string as raw JavaScript on the database server. An attacker can pass "0'; while(true){}" as the threshold and hang the database with an infinite loop, or pass a boolean-injection payload to bypass the filter entirely and match every document.

This is the real query from a deliberately-vulnerable Node.js reference app used for security training:

return {
  $where: `this.userId == ${parsedUserId} && this.stocks > '${threshold}'`
};

Your task: fix solve(userId, threshold) so it returns a filter object built from standard MongoDB query operators instead of an interpolated $where string:

// Expected return shape
{ userId: userId, stocks: { $gt: threshold } }