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

k8s/charts/seaweedfs/templates/volume/volume-statefulset.yaml

@@ -196,7 +196,7 @@ spec:
                 -minFreeSpacePercent={{ $volume.minFreeSpacePercent }} \
                 -ip=${POD_NAME}.${SEAWEEDFS_FULLNAME}-{{ $volumeName }}.{{ $.Release.Namespace }} \
                 -compactionMBps={{ $volume.compactionMBps }} \
-                -mserver={{ if $.Values.global.masterServer }}{{ $.Values.global.masterServer}}{{ else }}{{ range $index := until ($.Values.master.replicas | int) }}${SEAWEEDFS_FULLNAME}-master-{{ $index }}.${SEAWEEDFS_FULLNAME}-master.{{ $.Release.Namespace }}:{{ $.Values.master.port }}{{ if lt $index (sub ($.Values.master.replicas | int) 1) }},{{ end }}{{ end }}{{ end }}
+                -master={{ include "seaweedfs.masterServerArg" $ }} \
                 {{- range $volume.extraArgs }}
                 {{ . }} \
                 {{- end }}