update_assigned_rollup treated a failed count_assigned as zero and issued
ClearTodo, deleting an acked rollup todo on every transient forge error
(and, on a one-sided failure, corrupting the breakdown for one poll).
count_assigned now returns Result<u64, String> instead of Option<u64> so
the caller can log the cause, and decide_rollup only acts once both
counts are known — either failing leaves the existing todo state as-is
and logs a warn! naming which poll failed.
Closes#4720
ee25b7de20