feat: add RHI abstraction layer unit tests

- Add RHITestFixture with RHI_BACKEND env var support for backend selection
- Add unit tests for: Device, Buffer, Texture, SwapChain, CommandList, CommandQueue, Shader, Fence, Sampler
- Tests can run against D3D12 or OpenGL backends via RHI_BACKEND env var
- Add integration folder placeholder for future integration tests

tests/RHI/unit/fixtures/RHITestFixture.cpp

@@ -0,0 +1,47 @@
+#include "RHITestFixture.h"
+#include <cstdlib>
+#include <iostream>
+
+namespace XCEngine {
+namespace RHI {
+
+RHIType RHITestFixture::mBackendType = RHIType::D3D12;
+
+void RHITestFixture::SetUpTestSuite() {
+}
+
+void RHITestFixture::TearDownTestSuite() {
+}
+
+void RHITestFixture::SetUp() {
+    const char* backend = std::getenv("RHI_BACKEND");
+    if (backend != nullptr) {
+        std::string backendStr(backend);
+        if (backendStr == "OpenGL" || backendStr == "opengl") {
+            mBackendType = RHIType::OpenGL;
+        } else if (backendStr == "D3D12" || backendStr == "d3d12") {
+            mBackendType = RHIType::D3D12;
+        }
+    }
+
+    mDevice = RHIFactory::CreateRHIDevice(mBackendType);
+    ASSERT_NE(mDevice, nullptr);
+
+    RHIDeviceDesc desc = {};
+    desc.enableDebugLayer = true;
+    desc.appName = L"RHIUnitTest";
+
+    bool initResult = mDevice->Initialize(desc);
+    ASSERT_TRUE(initResult);
+}
+
+void RHITestFixture::TearDown() {
+    if (mDevice != nullptr) {
+        mDevice->Shutdown();
+        delete mDevice;
+        mDevice = nullptr;
+    }
+}
+
+} // namespace RHI
+} // namespace XCEngine