feat: add delete rows to table stats (#3783)

add to table stats the number of rows that has
been marked for deletion, but not yet deleted.
note that when a table fragment is removed due to
deletion of all of its rows, these rows are not counted. since the main
use of this feature is to determine when table compaction is needed, the
fact that these rows are not reported should not impact that decision
(deleted table fragments don't need compaction).

Fixes: https://github.com/lancedb/lancedb/issues/3761

Co-authored-by: Claude Opus 5.0 <noreply@anthropic.com>

python/src/table.rs

@@ -1266,6 +1266,7 @@ impl Table {
                 let dict = PyDict::new(py);
                 dict.set_item("total_bytes", stats.total_bytes)?;
                 dict.set_item("num_rows", stats.num_rows)?;
+                dict.set_item("num_deleted_rows", stats.num_deleted_rows)?;
                 dict.set_item("num_indices", stats.num_indices)?;
 
                 let fragment_stats = PyDict::new(py);