When `force_capacity_based_balancing` is enabled and a node is being drained/excluded, the tablet allocator incorrectly aborts balancing due to incomplete tablet stats - even though capacity-based balancing doesn't depend on tablet sizes.
The tablet allocator normally waits for complete load stats before balancing. An exception exists for drained+excluded nodes (they're unreachable and won't return stats). However, when forced capacity-based balancing is active, this exception was not being applied, causing the balancer to reject the drain plan.
Adjust the condition in `tablet_allocator.cc` so that the "ignore missing data for drained nodes" logic applies regardless of whether capacity-based balancing is forced.
Added a Boost unit test that forces capacity-based balancing and verifies a drained/excluded node gets its tablets migrated even when tablet size stats are missing.
This bug was introduced in 2026.1, so this needs to be backported to 2026.1 and 2026.2
Fixes: SCYLLADB-1803
Closesscylladb/scylladb#29791
* github.com:scylladb/scylladb:
test: boost: add drain test for forced capacity-based balancing
service: allow draining with forced capacity-based balancing
ddb1181103