Fix W5100S socket exhaustion blocking MQTT and additional TCP clients (#9770)

The W5100S Ethernet chip has only 4 hardware sockets. On RAK4631
Ethernet gateways with syslog and NTP enabled, all 4 sockets were
permanently consumed (NTP UDP + Syslog UDP + TCP API listener + TCP
API client), leaving none for MQTT, DHCP lease renewal, or additional
TCP connections.

- NTP: Remove permanent timeClient.begin() at startup; NTPClient::update()
  auto-initializes when needed. Add timeClient.end() after each query to
  release the UDP socket immediately.
- Syslog: Remove socket allocation from Syslog::enable(). Open and close
  the UDP socket on-demand in _sendLog() around each message send.
- MQTT: Fix socket leak in isValidConfig() where a successful test
  connection was never closed (PubSubClient destructor does not call
  disconnect). Add explicit pubSub->disconnect() before returning.

Made-with: Cursor

Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

src/mesh/eth/ethClient.cpp

@@ -102,7 +102,6 @@ static int32_t reconnectETH()
 
 #ifndef DISABLE_NTP
             LOG_INFO("Start NTP time client");
-            timeClient.begin();
             timeClient.setUpdateInterval(60 * 60); // Update once an hour
 #endif
 
@@ -159,6 +158,7 @@ static int32_t reconnectETH()
             LOG_ERROR("NTP Update failed");
             ntp_renew = millis() + 300 * 1000; // failure, retry every 5 minutes
         }
+        timeClient.end(); // W5100S: release UDP socket for other services
     }
 #endif