Add unit tests for printer driver, usb device and usb device factory

src/printers/P700Printer.cpp

@@ -90,6 +90,11 @@ libusbwrap::usbId P700Printer::getUsbId() {
 }
 
 bool P700Printer::attachUsbDevice(std::shared_ptr<libusbwrap::IUsbDevice> usbHndl) {
+    if (!usbHndl) {
+        spdlog::error("Cannot attach null USB device");
+        return false;
+    }
+
     if (!usbHndl->open()) {
         spdlog::error("Unable to open USB device: {}", usbHndl->getLastErrorString());
         return false;
@@ -182,6 +187,11 @@ bool P700Printer::printMonochromeData(const graphics::MonochromeData& data) {
 }
 
 bool P700Printer::printLabel(std::unique_ptr<graphics::ILabel> label) {
+    if (!label) {
+        spdlog::error("Cannot print null label");
+        return false;
+    }
+
     // Convert label directly to MonochromeData
     // getRaw() returns data in Cairo surface coordinates matching getWidth() × getHeight()
     auto pixels = label->getRaw();
@@ -200,9 +210,15 @@ bool P700Printer::printLabel(std::unique_ptr<graphics::ILabel> label) {
 }
 
 bool P700Printer::print() {
-    send(p700::commands::LF);
-    send(p700::commands::FF);
-    send(p700::commands::EJECT);
+    if (!send(p700::commands::LF)) {
+        return false;
+    }
+    if (!send(p700::commands::FF)) {
+        return false;
+    }
+    if (!send(p700::commands::EJECT)) {
+        return false;
+    }
     return true;
 }