fix: encapsulate OpenGL types in VertexAttribute to eliminate raw GL API usage in tests

- Add VertexAttributeType and VertexAttributeNormalized enums in OpenGLVertexArray.h
- Add ToGLAttributeType() converter in OpenGLVertexArray.cpp
- Remove glActiveTexture() call from quad test (already handled by texture.Bind())
- Remove #include <glad/glad.h> from triangle test
- Update unit tests to use encapsulated enums

All three OpenGL integration tests (minimal, triangle, quad) pass with 0% pixel difference.

docs/api/resources/audio-loader/get-default-settings.md

@@ -0,0 +1,27 @@
+# AudioLoader::GetDefaultSettings
+
+## 方法签名
+
+```cpp
+ImportSettings* GetDefaultSettings() const override;
+```
+
+## 详细描述
+
+返回音频资源的默认导入设置。当前版本音频加载器不使用导入设置，始终返回 `nullptr`。
+
+## 返回值
+
+`ImportSettings*` - 始终返回 `nullptr`。
+
+## 示例
+
+```cpp
+AudioLoader loader;
+ImportSettings* settings = loader.GetDefaultSettings();
+
+// 始终为 nullptr
+if (settings == nullptr) {
+    printf("No default settings for AudioLoader\n");
+}
+```