Fail test_conversation_from_postgres fast when DynamoDB is unavailable

Add require_dynamodb() guard so the test fails in ~3s instead of
hanging for minutes on a TCP connection to a stopped DynamoDB.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jucor

delphi/tests/test_postgres_real_data.py

@@ -521,6 +521,9 @@ def test_conversation_from_postgres():
     """
     Test processing a conversation with data from PostgreSQL.
     """
+    from tests.conftest import require_dynamodb
+    require_dynamodb()
+
     import time
     start_time = time.time()