source-mongodb: require sorting on _id for batch captures

The assumption that documents are returned in an ascending insertion order does
not appear to be true for non-standard MongoDB deployments. These kinds of
deployments don't support change streams and must be captured in a batch mode,
so for bindings that use batch mode always require an explicit sort.

source-mongodb/backfill.go

@@ -206,12 +206,16 @@ func (c *capture) doBackfill(
 
  cursorField := binding.resource.getCursorField()
  var opts *options.FindOptions
- if cursorField == idProperty {
+ if cursorField == idProperty && binding.resource.getMode() == captureModeChangeStream {
  // By not specifying a sort parameter, MongoDB uses natural sort to order documents. Natural
  // sort is approximately insertion order (but not guaranteed). We hint to MongoDB to use the _id
  // index (an index that always exists) to speed up the process. Note that if we specify the sort
  // explicitly by { $natural: 1 }, then the database will disregard any indices and do a full
  // collection scan. See https://www.mongodb.com/docs/manual/reference/method/cursor.hint
+
+ // Note: This assumption is only true for "standard" MongoDB instances that support change
+ // streams. For other flavors of MongoDB that do not support change streams and/or we are using
+ // a batch capture mode, a sort will be required.
  opts = options.Find().SetHint(bson.M{idProperty: 1})
  } else {
  // Other cursor fields require a sort.
@@ -339,7 +343,7 @@ func (c *capture) pullCursor(
  var doc bson.M
  var err error
  if lastCursor, err = cursor.Current.LookupErr(cursorField); err != nil {
- return 0, fmt.Errorf("looking up idProperty: %w", err)
+ return 0, fmt.Errorf("looking up cursor field '%s': %w", cursorField, err)
  } else if err = cursor.Decode(&doc); err != nil {
  return 0, fmt.Errorf("backfill decoding document: %w", err)
  }