refactor: remove unnecessary KeepMasterClientConnected wrapper in filer

The Filer.KeepMasterClientConnected() method was an unnecessary wrapper that
just forwarded to MasterClient.KeepConnectedToMaster(). This wrapper added
no value and created inconsistency with other components that call
KeepConnectedToMaster directly.

Removed:
  filer.go:178-180
    func (fs *Filer) KeepMasterClientConnected(ctx context.Context) {
        fs.MasterClient.KeepConnectedToMaster(ctx)
    }

Updated caller:
  filer_server.go:181
    - go fs.filer.KeepMasterClientConnected(context.Background())
    + go fs.filer.MasterClient.KeepConnectedToMaster(context.Background())

Benefits:
  - Consistent with other components (S3, IAM, Shell, Mount)
  - Removes unnecessary indirection
  - Clearer that KeepConnectedToMaster runs in background goroutine
  - Follows the documented pattern from MasterClient.GetMaster()

Note: shell/commands.go was verified and already correctly starts
KeepConnectedToMaster in a background goroutine (shell_liner.go:51):
  go commandEnv.MasterClient.KeepConnectedToMaster(ctx)

weed/filer/filer.go

@@ -175,10 +175,6 @@ func (fs *Filer) GetMaster(ctx context.Context) pb.ServerAddress {
 	return fs.MasterClient.GetMaster(ctx)
 }
 
-func (fs *Filer) KeepMasterClientConnected(ctx context.Context) {
-	fs.MasterClient.KeepConnectedToMaster(ctx)
-}
-
 func (f *Filer) BeginTransaction(ctx context.Context) (context.Context, error) {
 	return f.Store.BeginTransaction(ctx)
 }

weed/server/filer_server.go

@@ -178,7 +178,7 @@ func NewFilerServer(defaultMux, readonlyMux *http.ServeMux, option *FilerOption)
 	fs.checkWithMaster()
 
 	go stats.LoopPushingMetric("filer", string(fs.option.Host), fs.metricsAddress, fs.metricsIntervalSec)
-	go fs.filer.KeepMasterClientConnected(context.Background())
+	go fs.filer.MasterClient.KeepConnectedToMaster(context.Background())
 
 	fs.option.recursiveDelete = v.GetBool("filer.options.recursive_delete")
 	v.SetDefault("filer.options.buckets_folder", "/buckets")