fix(server_common, wsbroadcastserver): correct machine callback condition and add write deadline

[maximevtush]

Description

• Summary: Ensure ForEachReadyMachine invokes callback only on successful machine retrieval; add bounded write deadline for client writes.
• Problem:
  • validator/server_common/machine_loader.go: callback executed when stat.Current() returned an error.
  • wsbroadcastserver/clientconnection.go: conn.Write could block indefinitely on slow/stalled clients.
• Solution:
  • Invert condition to call callback only if err == nil.
  • Set and reset a write deadline (10s) around conn.Write.