…AssignedPartitions()`

Fixes spring-projects#4273

The previous fix (spring-projectsGH-3726) wrapped `definedPartitions` and `assignedPartitions`
with `Collections.synchronizedMap/Set`, but this only synchronizes individual
operations. Per Java documentation, manual synchronization is required when
iterating over synchronized collection views.

The method was returning unmodifiable views of these collections, which
callers would then iterate outside any synchronization. When partitions
were reassigned concurrently (e.g., during rebalance), the iteration would
fail with `ConcurrentModificationException`.

The fix synchronizes on the underlying collection while creating a snapshot
copy. This ensures the iteration (during copy) is protected, and callers
receive an independent snapshot safe to use without holding any locks.

Additionally, `wrapUp()` now explicitly passes an empty collection to
`onPartitionsRevoked()` during shutdown. Previously, this accidentally
worked because the mutable view would become empty after `unsubscribe()`
cleared the underlying collection. Since `getAssignedPartitions()` now
returns a snapshot, we must explicitly pass an empty collection to preserve
the shutdown signal semantics. The actual partition cleanup already occurs
via the rebalance listener during `unsubscribe()`.

Signed-off-by: Soby Chacko <soby.chacko@broadcom.com>