fix: add missing backslash for volume extraArgs in helm chart (#7676)

Fixes #7467

The -mserver argument line in volume-statefulset.yaml was missing a
trailing backslash, which prevented extraArgs from being passed to
the weed volume process.

Also:
- Extracted master server list generation logic into shared helper
  templates in _helpers.tpl for better maintainability
- Updated all occurrences of deprecated -mserver flag to -master
  across docker-compose files, test files, and documentation

test/erasure_coding/ec_integration_test.go

@@ -431,7 +431,7 @@ func startSeaweedFSCluster(ctx context.Context, dataDir string) (*TestCluster, e
 			"-port", port,
 			"-dir", volumeDir,
 			"-max", "10",
-			"-mserver", "127.0.0.1:9333",
+			"-master", "127.0.0.1:9333",
 			"-ip", "127.0.0.1",
 			"-dataCenter", "dc1",
 			"-rack", rack,
@@ -956,7 +956,7 @@ func startMultiDiskCluster(ctx context.Context, dataDir string) (*MultiDiskClust
 			"-port", port,
 			"-dir", strings.Join(diskDirs, ","),
 			"-max", strings.Join(maxVolumes, ","),
-			"-mserver", "127.0.0.1:9334",
+			"-master", "127.0.0.1:9334",
 			"-ip", "127.0.0.1",
 			"-dataCenter", "dc1",
 			"-rack", rack,