feat(automation): add open_files to backend interface + kErrLoadFailed (1007)

src/slic3r/GUI/Automation/IUiBackend.hpp

@@ -95,6 +95,12 @@ public:
     // Screenshot. target == nullptr => main frame. Captured from the on-screen
     // composited framebuffer, so it includes the GL viewport and ImGui overlays.
     virtual PngImage screenshot_window(const UiNode* target) = 0;
+
+    // Load one or more files (absolute paths) into the running instance on the GUI
+    // thread. Returns the number of objects added to the scene (load_files(...).size()).
+    // Throws AutomationError(kErrLoadFailed) when nothing loads. Header stays wx-free:
+    // the concrete LoadStrategy is chosen inside WxUiBackend, not exposed here.
+    virtual int open_files(const std::vector<std::string>& paths) = 0;
 };
 
 }}} // namespace Slic3r::GUI::Automation

src/slic3r/GUI/Automation/JsonRpcDispatcher.hpp

@@ -17,6 +17,7 @@ constexpr int kErrWaitTimeout     = 1003;
 constexpr int kErrGuiBusy         = 1004; // GUI thread timeout
 constexpr int kErrScreenshotFail  = 1005;
 constexpr int kErrDisabled        = 1006;
+constexpr int kErrLoadFailed      = 1007; // file.open: load_files returned empty / threw
 
 constexpr const char* kProtocolVersion = "2.0";
 constexpr const char* kAutomationVersion = "1.0.0";