feat(tests): Add stub config service and enhance bgfx_gui_service tests

src/services/impl/bgfx_gui_service.cpp

@@ -243,6 +243,14 @@ void BgfxGuiService::Shutdown() noexcept {
     initialized_ = false;
 }
 
+bool BgfxGuiService::IsProgramReady() const {
+    return bgfx::isValid(program_);
+}
+
+bool BgfxGuiService::IsWhiteTextureReady() const {
+    return bgfx::isValid(whiteTexture_);
+}
+
 void BgfxGuiService::InitializeResources() {
     if (initialized_) {
         return;

src/services/impl/bgfx_gui_service.hpp

@@ -32,6 +32,9 @@ public:
 
     void Shutdown() noexcept override;
 
+    bool IsProgramReady() const;
+    bool IsWhiteTextureReady() const;
+
 private:
     struct GuiVertex {
         float x = 0.0f;