Add error handling for unknown transaction modes in TransactionSettings.to_proto()

Co-authored-by: asmyasnikov <14202262+asmyasnikov@users.noreply.github.com>

Author: Copilot

ydb/_grpc/grpcwrapper/ydb_query.py

@@ -81,6 +81,7 @@ def to_proto(self) -> ydb_query_pb2.TransactionSettings:
             return ydb_query_pb2.TransactionSettings(stale_read_only=self.tx_mode.to_proto())
         if self.tx_mode.name == "snapshot_read_write":
             return ydb_query_pb2.TransactionSettings(snapshot_read_write=self.tx_mode.to_proto())
+        raise ValueError(f"Unknown transaction mode: {self.tx_mode.name}")
 
 
 @dataclass