command: implement graceful shutdown for mini cluster

- Introduce MiniClusterCtx to coordinate shutdown across mini services
- Update Master, Volume, Filer, S3, and WebDAV servers to respect context cancellation
- Ensure all resources are cleaned up properly during test teardown
- Integrate MiniClusterCtx in s3tables integration tests

weed/command/master.go

@@ -311,7 +311,13 @@ func startMaster(masterOption MasterOptions, masterWhiteList []string) {
 			ms.Topo.HashicorpRaft.LeadershipTransfer()
 		}
 	})
-	select {}
+	if MiniClusterCtx != nil {
+		<-MiniClusterCtx.Done()
+		ms.Shutdown()
+		grpcS.Stop()
+	} else {
+		select {}
+	}
 }
 
 func isSingleMasterMode(peers string) bool {