feat(balance): replica placement validation for volume moves (#8622)

* feat(balance): add replica placement validation for volume moves

When the volume balance detection proposes moving a volume, validate
that the move does not violate the volume's replication policy (e.g.,
ReplicaPlacement=010 requires replicas on different racks). If the
preferred destination violates the policy, fall back to score-based
planning; if that also violates, skip the volume entirely.

- Add ReplicaLocation type and VolumeReplicaMap to ClusterInfo
- Build replica map from all volumes before collection filtering
- Port placement validation logic from command_volume_fix_replication.go
- Thread replica map through collectVolumeMetrics call chain
- Add IsGoodMove check in createBalanceTask before destination use

* address PR review: extract validation closure, add defensive checks

- Extract validateMove closure to eliminate duplicated ReplicaLocation
  construction and IsGoodMove calls
- Add defensive check for empty replica map entries (len(replicas) == 0)
- Add bounds check for int-to-byte cast on ExpectedReplicas (0-255)

* address nitpick: rp test helper accepts *testing.T and fails on error

Prevents silent failures from typos in replica placement codes.

* address review: add composite replica placement tests (011, 110)

Test multi-constraint placement policies where both rack and DC
rules must be satisfied simultaneously.

* address review: use struct keys instead of string concatenation

Replace string-concatenated map keys with typed rackKey/nodeKey
structs to eliminate allocations and avoid ambiguity if IDs
contain spaces.

* address review: simplify bounds check, log fallback error, guard source

- Remove unreachable ExpectedReplicas < 0 branch (outer condition
  already guarantees > 0), fold bounds check into single condition
- Log error from planBalanceDestination in replica validation fallback
- Return false from IsGoodMove when sourceNodeID not found in
  existing replicas (inconsistent cluster state)

* address review: use slices.Contains instead of hand-rolled helpers

Replace isAmongDC and isAmongRack with slices.Contains from the
standard library, reducing boilerplate.

weed/plugin/worker/volume_balance_handler.go

@@ -314,7 +314,7 @@ func (h *VolumeBalanceHandler) Detect(
 		masters = append(masters, request.ClusterContext.MasterGrpcAddresses...)
 	}
 
-	metrics, activeTopology, err := h.collectVolumeMetrics(ctx, masters, collectionFilter)
+	metrics, activeTopology, replicaMap, err := h.collectVolumeMetrics(ctx, masters, collectionFilter)
 	if err != nil {
 		return err
 	}
@@ -334,7 +334,10 @@ func (h *VolumeBalanceHandler) Detect(
 	workerConfig.TaskConfig.RackFilter = rackFilter
 	workerConfig.TaskConfig.NodeFilter = nodeFilter
 
-	clusterInfo := &workertypes.ClusterInfo{ActiveTopology: activeTopology}
+	clusterInfo := &workertypes.ClusterInfo{
+		ActiveTopology:   activeTopology,
+		VolumeReplicaMap: replicaMap,
+	}
 	maxResults := int(request.MaxResults)
 
 	var results []*workertypes.TaskDetectionResult
@@ -1072,10 +1075,8 @@ func (h *VolumeBalanceHandler) collectVolumeMetrics(
 	ctx context.Context,
 	masterAddresses []string,
 	collectionFilter string,
-) ([]*workertypes.VolumeHealthMetrics, *topology.ActiveTopology, error) {
-	// Reuse the same master topology fetch/build flow used by the vacuum handler.
-	helper := &VacuumHandler{grpcDialOption: h.grpcDialOption}
-	return helper.collectVolumeMetrics(ctx, masterAddresses, collectionFilter)
+) ([]*workertypes.VolumeHealthMetrics, *topology.ActiveTopology, map[uint32][]workertypes.ReplicaLocation, error) {
+	return collectVolumeMetricsFromMasters(ctx, masterAddresses, collectionFilter, h.grpcDialOption)
 }
 
 func deriveBalanceWorkerConfig(values map[string]*plugin_pb.ConfigValue) *volumeBalanceWorkerConfig {