How to read length-prefixed binary message from Serial using Arduino

#include <Arduino.h>

void setup() {
    Serial.begin(115200);
}

void HandleMessage(String msg) {
    // TODO: Your code to handle the message goes here.
    // See https://techoverflow.net/2022/11/15/how-to-print-string-as-sequence-of-hex-bytes-in-arduino/
    // for an example of how to print the message as a sequence of hex bytes.
}

void ReadMessageFromSerial() {
    // Wait until the length byte is available on Serial
    while (Serial.available() == 0);

    // Read the length of the message
    int length = Serial.read();

    // Read the rest of the message
    String message = "";
    for (int i = 0; i < length; i++) {
      while (Serial.available() == 0);
      message += char(Serial.read());
    }

    // Handle the message
    HandleMessage(message);
}

void loop() {
    ReadMessageFromSerial();
}