SIENTIAPDE-1646

Harden retrain merge and OPC write against silent failures

- pandas merge now uses validate='one_to_one' so duplicated timestamps
  from wrapper.retrain fail loudly instead of silently multiplying rows
- OPC write_data raises OpcClientNotInitializedError instead of a bare
  assert when client is torn down between validation and write, so
  callers get a reported error instead of an AssertionError
- add regression test covering the torn-down client path

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/laborious/utils/repository/test_opc_repository.py

@@ -444,6 +444,22 @@ def test_write_data_get_node_failed(opc_repository):
     assert error_data['attachment_content'] is not None
 
 
+def test_write_data_without_client_fails_as_get_node_error(opc_repository):
+    """A client torn down between validation and the write is reported, not asserted away."""
+    opc_repository.validate_connection = MagicMock(return_value=(True, {}))
+    opc_repository.client = None
+
+    is_success, error_data = opc_repository.write_data(
+        'ns=2;s=TestNode', 42.0, 'float', metadata['metadata']
+    )
+
+    assert is_success is False
+    assert error_data['notification_id'] == f'OPC_WRITE_GET_NODE_ERROR_{opc_repository.id}'
+    assert 'OPC client is not initialized' in error_data['message']
+    last_call = opc_repository.emit_metric_sync.call_args_list[-1]
+    assert last_call.args[1]['result'] == 'GetNodeError:OpcClientNotInitializedError'
+
+
 def test_write_data_invalid_data_type(opc_repository, mock_client):
     opc_repository.validate_connection = MagicMock(return_value=(True, {}))
     opc_repository.client = mock_client