I'm sorry if I'm being obtuse, but the sketch works perfectly with the string as included in the sketch:
String codeString = "import machine\nimport utime\n\ngpio0 = machine.Pin(0, machine.Pin.OUT)\ngpio45 = machine.Pin(45, machine.Pin.OUT)\nutime.sleep(1)\ngpio45.value(0)\ngpio0.value(1)";
Serial.println(codeString);
This gives the following in the monitor:
import machine
import utime
gpio0 = machine.Pin(0, machine.Pin.OUT)
gpio45 = machine.Pin(45, machine.Pin.OUT)
utime.sleep(1)
gpio45.value(0)
gpio0.value(1)
Which is what I want from the input string "import machine\nimport utime\n\ngpio0 = machine.Pin(0, machine.Pin.OUT)\ngpio45 = machine.Pin(45, machine.Pin.OUT)\nutime.sleep(1)\ngpio45.value(0)\ngpio0.value(1)"