issue: #45598
The MultiTargetBalancer was missing replica field assignment in the
generated segment and channel plans, which caused panic during balance
operations. This change ensures that all balance plans have the replica
field properly set to fix the panic issue.
Also refactored the balance test to extract common test logic into a
reusable helper function and added a new integration test specifically
for MultipleTargetBalancer policy.
Signed-off-by: Wei Liu <wei.liu@zilliz.com>
a308331b81