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.h

@@ -0,0 +1,39 @@
+#pragma once
+
+#include <gtest/gtest.h>
+#include <string>
+
+#include "XCEngine/RHI/RHIFactory.h"
+#include "XCEngine/RHI/RHIDevice.h"
+#include "XCEngine/RHI/RHICommandQueue.h"
+#include "XCEngine/RHI/RHICommandList.h"
+#include "XCEngine/RHI/RHIBuffer.h"
+#include "XCEngine/RHI/RHITexture.h"
+#include "XCEngine/RHI/RHISwapChain.h"
+#include "XCEngine/RHI/RHIShader.h"
+#include "XCEngine/RHI/RHIFence.h"
+#include "XCEngine/RHI/RHISampler.h"
+#include "XCEngine/RHI/RHIDescriptorPool.h"
+#include "XCEngine/RHI/RHIPipelineLayout.h"
+
+namespace XCEngine {
+namespace RHI {
+
+class RHITestFixture : public ::testing::Test {
+protected:
+    void SetUp() override;
+    void TearDown() override;
+
+    static void SetUpTestSuite();
+    static void TearDownTestSuite();
+
+    RHIDevice* GetDevice() { return mDevice; }
+    RHIType GetBackendType() const { return mBackendType; }
+
+private:
+    static RHIType mBackendType;
+    RHIDevice* mDevice = nullptr;
+};
+
+} // namespace RHI
+} // namespace XCEngine