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/sftp/framework.go

@@ -268,7 +268,7 @@ func (f *SftpTestFramework) startMaster(config *TestConfig) error {
 func (f *SftpTestFramework) startVolumeServer(config *TestConfig) error {
 	args := []string{
 		"volume",
-		"-mserver=" + f.masterAddr,
+		"-master=" + f.masterAddr,
 		"-ip=127.0.0.1",
 		"-port=18080",
 		"-dir=" + filepath.Join(f.dataDir, "volume"),