Remove newline requirement for serial json parsing

[cjsha]

• Rewrite serial buffer builder to not need newline termination:
  • Let ArduinoJson decide if serial buffer is terminated
  • Ignore characters preceding {
    • This prevents DeserializeJson from parsing strings or arrays instead of JSON objects (i.e. handles the case when a double quotes " or open square bracket [ precedes your JSON which would otherwise brick serial inputs until the string or array is terminated)
    • An alternative is to rollback ArduinoJson to v5 and instead use the ParseObject method
  • Restart buffer if new { is received
    • This prevents a singular open curly bracket '{' from bricking serial inputs. This is a proposed improvement from

---

I tried using std::cin instead of constructing our own buffer. There were some problems associated with that.

Originally, the process_serial_commands was only called when a new character was received from the serial port. This meant that if I sent a message like "a{turn:1}" or "{enable:true}{turn:1}", only the first item would be parses (in the first case, an invalid input "a"; in the second case, a valid JSON), and the second item wouldn't be parsed until another character was received. If I remove the guard, std::cin would block when it's empty, and there doesn't seem to be any standard way to check if std::cin is empty without blocking.

[jonnew]

There needs to be something that flushes standard in an resets the serial_buffer when the remote_avialabel transitions from low to high, e.g.

static int process_serial_commands(context_t *ctx)
{
    static char serial_buffer[MAX_SERIAL_BUFFER_LENGTH] = {0};
    static uint16_t serial_buffer_index = 0;
    static bool last_remote_available = false;

    if (!last_remote_available && remote_available(ctx))
    {
        // TODO: reset buffer and flush stdin
        last_remote_available = true;

    } else {
        last_remote_available = false;
        return ERROR_REMOTE_INTERFACE_LOCKED;
    }

or perhaps something a bit less dumb