filterQuery left $select, $or, $and, and $sort object values
unvalidated, so unknown $ operators could ride through those
filters. ObjectIdSchema accepted any object, which let operator
documents pass querySyntax on generated MongoDB services.

Validate those filter values with validateQueryProperty. The
objectid keyword now accepts ObjectId instances and rejects
other objects, and ObjectIdSchema uses that for its object branch.

Tightening ObjectIdSchema dropped { _id: { $exists: true } }, which
only passed because the old any-object branch treated the operator
document as an id. Add $exists as a boolean operator on queryProperty
so it is allowed on every querySyntax field, including ObjectIds.

{ userId: { $ne: null } } is the usual Mongo and SQL "field is set"
query. After tightening ObjectIdSchema it only passed when the field
type already included null. Accept null on $ne/$in/$nin in
queryProperty so ObjectId, number, and string fields all allow it.

The default syntax already covers $ne/$in/$nin null and $exists.
Document how to add $like/$regex/array operators, how to allow
{ userId: null } on ObjectId query fields, and how $meta/$slice
map to service operators rather than querySyntax.

{ userId: null } is a real IS NULL query; accept null on the
queryProperty equality branch the same way as $ne/$in/$nin.

Allow $meta/$slice/$elemMatch inside object $select and $sort
(not as field operators). Add more with projectionOperators.

@feathersjs/mongodb defaults operators to include $regex and
$options. Generated Mongo query schemas do the same on the
primary string field.

Keep the allow-list tightenings. Remove $exists, implicit null
equality, default Mongo $regex, and projectionOperators so those
stay app-level extensions via querySyntax or operators, not new
supported syntax.

Assert $regex/$options, $like, $exists, $meta, and nullable
ObjectId fields still work when the app opts in, and that
unlisted operators such as $where stay rejected.

isObjectId now requires instanceof ObjectId, or _bsontype plus a
non-Object constructor and toHexString, so `{ _bsontype: 'ObjectId' }`
does not pass ObjectIdSchema. Attach $regex/$options to text in the
TypeBox Pick example.