fix(s3): start KeepConnectedToMaster for filer discovery with filerGroup (#7732)

Fixes #7721

When S3 server is configured with a filerGroup, it creates a MasterClient
to enable dynamic filer discovery. However, the KeepConnectedToMaster()
background goroutine was never started, causing GetMaster() to block
indefinitely in WaitUntilConnected().

This resulted in the log message:
  WaitUntilConnected still waiting for master connection (attempt N)...

being logged repeatedly every ~20 seconds.

The fix adds the missing 'go masterClient.KeepConnectedToMaster(ctx)'
call to properly establish the connection to master servers.

Also adds unit tests to verify WaitUntilConnected respects context
cancellation.

weed/s3api/s3api_server.go

@@ -115,6 +115,8 @@ func NewS3ApiServerWithStore(router *mux.Router, option *S3ApiServerOption, expl
 			masterMap[fmt.Sprintf("master%d", i)] = addr
 		}
 		masterClient := wdclient.NewMasterClient(option.GrpcDialOption, option.FilerGroup, cluster.S3Type, "", "", "", *pb.NewServiceDiscoveryFromMap(masterMap))
+		// Start the master client connection loop - required for GetMaster() to work
+		go masterClient.KeepConnectedToMaster(context.Background())
 
 		filerClient = wdclient.NewFilerClient(option.Filers, option.GrpcDialOption, option.DataCenter, &wdclient.FilerClientOption{
 			MasterClient:      masterClient,