fix(test): avoid port collision between master gRPC and volume ports

AllocateMiniPorts(1) reserved masterPort and masterPort+GrpcPortOffset
by holding listeners open, but closed them on return. The subsequent
AllocatePorts call bound 127.0.0.1:0, so the OS could immediately reuse
the just-released mini gRPC port as a volume port — causing the volume
server to fail at bind time with "address already in use".

Introduce AllocatePortSet(miniCount, regularCount) that holds every
listener open until the full set is chosen, and route the five volume
test cluster builders through it.

test/volume_server/framework/cluster_multi_rust.go

@@ -86,13 +86,6 @@ func StartRustMultiVolumeCluster(t testing.TB, profile matrix.Profile, serverCou
 		t.Fatalf("write security config: %v", err)
 	}
 
-	miniPorts, err := testutil.AllocateMiniPorts(1)
-	if err != nil {
-		t.Fatalf("allocate master port pair: %v", err)
-	}
-	masterPort := miniPorts[0]
-	masterGrpcPort := masterPort + testutil.GrpcPortOffset
-
 	// Allocate ports for all volume servers (3 ports per server: admin, grpc, public)
 	// If SplitPublicPort is true, we need an additional port per server
 	portsPerServer := 3
@@ -100,10 +93,12 @@ func StartRustMultiVolumeCluster(t testing.TB, profile matrix.Profile, serverCou
 		portsPerServer = 4
 	}
 	totalPorts := serverCount * portsPerServer
-	ports, err := testutil.AllocatePorts(totalPorts)
+	miniPorts, ports, err := testutil.AllocatePortSet(1, totalPorts)
 	if err != nil {
-		t.Fatalf("allocate volume ports: %v", err)
+		t.Fatalf("allocate ports: %v", err)
 	}
+	masterPort := miniPorts[0]
+	masterGrpcPort := masterPort + testutil.GrpcPortOffset
 
 	c := &RustMultiVolumeCluster{
 		testingTB:         t,