dekaf: Always return at least 2 docs for data preview UI requests

crates/dekaf/src/session.rs

@@ -491,7 +491,10 @@ impl Session {
                                     Some(partition_request.fetch_offset - 1),
                                 )
                                 .next_batch(
-                                    crate::read::ReadTarget::Docs(diff as usize + 1),
+                                    // Have to read at least 2 docs, as the very last doc
+                                    // will probably be a control document and will be
+                                    // ignored by the consumer, looking like 0 docs were read
+                                    crate::read::ReadTarget::Docs(max(diff as usize, 2)),
                                     std::time::Instant::now() + timeout,
                                 ),
                             )