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/kms/docker-compose.yml

@@ -64,7 +64,7 @@ services:
       - "8080:8080"
     command:
       - volume
-      - -mserver=seaweedfs-master:9333
+      - -master=seaweedfs-master:9333
       - -ip=seaweedfs-volume
       - -publicUrl=seaweedfs-volume:8080
     depends_on: