Fix Serial1.end() hang on nRF52: pull up RX before UART shutdown

When GPS powers down, its TX line drops LOW, putting the nRF52 UARTE
RX in a continuous break condition. Uart::end() waits for RXTO (receiver
timeout) which only fires after the receiver is idle for one character
period — never happening during a break. Result: infinite hang.

Fix: pull up the MCU's RX pin (PIN_GPS_TX = "toward CPU") before calling
Serial1.end(), ensuring the line is in idle state so RXTO fires promptly.

Author: weebl2000

src/helpers/sensors/EnvironmentSensorManager.cpp

@@ -583,6 +583,10 @@ void EnvironmentSensorManager::initBasicGPS() {
     MESH_DEBUG_PRINTLN("No GPS detected");
   }
   _location->stop();
+  // Pull up MCU's RX pin before ending UART. When GPS powers down, its TX
+  // line drops LOW (break condition) which prevents nRF52 UARTE's RXTO event
+  // from firing, causing Serial1.end() to hang indefinitely.
+  pinMode(PIN_GPS_TX, INPUT_PULLUP);  // PIN_GPS_TX = toward CPU = MCU's RX
   Serial1.end();  // Disable UART peripheral to save power (significant on nRF52)
   gps_active = false; //Set GPS visibility off until setting is changed
 }
@@ -706,6 +710,8 @@ void EnvironmentSensorManager::stop_gps() {
   #endif
 
   _location->stop();
+  // Pull up MCU's RX pin before ending UART - see comment in initBasicGPS()
+  pinMode(PIN_GPS_TX, INPUT_PULLUP);
   Serial1.end();  // Disable UART peripheral to save power (significant on nRF52)
 
   #ifndef PIN_GPS_EN