direct_consumer/api_types: fix invalid lso

Changes the initial state of last_stable_offset to be invalid_lso

Will research the proper 'invalid' offsets for the others in a followon.

Author: joe-redpanda

src/v/kafka/client/direct_consumer/api_types.h

@@ -92,7 +92,7 @@ struct source_partition_offsets {
     // The source partition's log high watermark
     kafka::offset high_watermark{-1};
     // The source partition's log last stable offset
-    kafka::offset last_stable_offset{-1};
+    kafka::offset last_stable_offset{model::offset_cast(model::invalid_lso)};
     // The timestamp that the fetch response was received by the client
     ss::lowres_clock::time_point last_offset_update_timestamp{};