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

docker/compose/swarm-etcd.yml

@@ -71,7 +71,7 @@ services:
       - net
     command:
       - 'volume'
-      - '-mserver=master:9333'
+      - '-master=master:9333'
       - '-port=8080'
     deploy:
       mode: global