fix: handle AirbyteDateTime objects in DatetimeBasedCursor

Co-Authored-By: Aaron <AJ> Steers <[email protected]>

Author: devin-ai-integration[bot]

airbyte_cdk/sources/declarative/incremental/datetime_based_cursor.py

@@ -294,6 +294,8 @@ def _evaluate_next_start_date_safely(
         would have broken anyway.
         """
         try:
+            if hasattr(start, "to_datetime"):
+                start = start.to_datetime()
             return start + step
         except OverflowError:
             return datetime.datetime.max.replace(tzinfo=datetime.timezone.utc)