During the initial implementation of the view builing coordinator,
we decided that if a view building task fails locally on the worker
(example reason: view update's target replica is not available),
the worker will retry this work instead of reporting a failure to the
coordinator.
However, we left return type of the RPC, which was telling if a task was
finished successfully or aborted.
But the worker doesn't need to report that a task was aborted, because
it's the coordinator, who decides to abort a task.
So, this commit changes the return type to list of UUIDs of completed
tasks.
Previously length of the returned vector needed to be the same as length
of the vector sent in the request.
No we can drop this restriction and the RPC handler return list of UUIDs
of completed tasks (subset of vector sent in the request).
This change is required to drop `STARTED` state in next commits.
Since Scylla 2025.4 wasn't released yet and we're going to merge this
patch before releasing, no RPC versioning or cluster feature is needed.
(cherry picked from commit 6d853c8f11)
8df72bb658