Use Case

As a MongoDB Node.js driver user
I want documents containing `_bsontype` as a plain data field to serialize correctly
So that documents inserted by other drivers can be read and written back without errors

User Experience

• A document like ` { _bsontype: "foo", value: 1 }

  ` stored by any driver deserializes to a plain JS object fine, but cannot be re-serialized. At root level it throws `BSONError: BSON types cannot be serialized as a document`; nested, it throws `BSONVersionError` because the plain deserialized object has no `@@mdb.bson.version` symbol. Read-modify-write on such documents is broken.

Dependencies

• The fix involves updating the root-level `_bsontype` guard (added in #537) and the nested version-symbol gate (added in #709) to use the `bsonType` symbol (`@@mdb.bson.type`) for dispatch instead of the `_bsontype` string property.
• `calculate_size.ts` dispatches on `_bsontype` as well and needs the same change.

Risks/Unknowns

• Lifting the root-level guard (a deliberate BSON v5 breaking change, #537) needs careful review to confirm real BSON type instances at the root still throw.
• Changing the nested gate from `value._bsontype == null` to `value[bsonType] == null` means objects with `_bsontype` as a plain string but no `bsonType` symbol will serialize as plain documents instead of throwing `BSONVersionError`. That is the desired fix, but it is a behavior change.

Acceptance Criteria

Implementation Requirements

• `bson.deserialize(bson.serialize(doc))` round-trips cleanly for documents with `_bsontype` as a plain string field, at root and nested.
• Real BSON type instances (ObjectId, Int32, etc.) at the root still throw.
• `calculateObjectSize` agrees with `serialize` for all affected cases.

Testing Requirements

• Round-trip ` { _bsontype: "anything" }

  ` at root and nested, confirm no throw.

• Confirm BSON type instances at root still throw.
• Existing serializer input-validation tests pass.

Documentation Requirements

• This is a breaking change - flag it in the release notes.

Follow Up Requirements

• N/A