Restore subscriptions after relay reconnects

[Copilot]

Subscriptions were lost when relay connections dropped, with no API to determine whether a subscription remained active. This change preserves subscription state and restores it automatically after reconnection.

• Subscription status
  • Add getSubscription() and hasSubscription() accessors.
• Reconnect recovery
  • Persist each subscription’s REQ payload and relay targets.
  • Requeue matching REQ messages when a relay reports CONNECTED.
  • Reset EOSE state so restored subscriptions receive fresh stored events.
• Tests
  • Cover subscription lookup, removal, and reconnect restoration.

if (pool.hasSubscription(subId)) {
    auto *subscription = pool.getSubscription(subId);
}

• Fixes Feature request: Add getter and autoreconnect for subscriptions #3