core-users: findUsers with `usernames`/`emails` throws MongoServerError "cannot nest $ under $in"

[pozylon]

Bug

buildFindSelector in packages/core-users/src/module/configureUsersModule.ts builds an invalid MongoDB query when usernames or emails are passed:

if (usernames?.length) {
  selector.username = {
    $in: usernames.map((u) => insensitiveTrimmedRegexOperator(u)),
  } as any;
}
if (emails?.length) {
  selector['emails.address'] = {
    $in: emails.map((e) => insensitiveTrimmedRegexOperator(e)),
  } as any;
}

insensitiveTrimmedRegexOperator (in @unchainedshop/mongodb) returns an operator document { $regex: RegExp }. MongoDB allows plain regex values inside $in, but not operator documents, so any findUsers({ usernames: [...] }) / findUsers({ emails: [...] }) call fails with:

MongoServerError: cannot nest $ under $in
  code: 2, codeName: 'BadValue'

Affects the released 4.8.22 — present on both the v4.8.x branch (configureUsersModule.ts L87–95) and master (L86–94). A patch on v4.8.x (→ 4.8.23) would be appreciated.

Repro

await modules.users.findUsers({ usernames: ['someuser'] });
// MongoServerError: cannot nest $ under $in

Suggested fix

Map to raw RegExp values instead of the { $regex } wrapper, e.g.:

$in: usernames.map((u) => insensitiveTrimmedRegexOperator(u).$regex)

or add a helper that returns the bare RegExp for use inside $in.