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/integration/README.md

@@ -0,0 +1,25 @@
+# RHI Integration Tests
+
+This folder will contain integration tests for the RHI abstraction layer.
+
+Integration tests verify the interaction between multiple RHI components working together.
+
+## Planned Tests
+
+- Full render pipeline test (device -> swap chain -> command list -> present)
+- Multi-threaded command recording
+- Resource state transitions across multiple objects
+- Frame synchronization with fences
+
+## Running Tests
+
+```bash
+# Build once, run against different backends
+cmake --build build --config Debug
+
+# Test D3D12 backend
+RHI_BACKEND=D3D12 ./build/tests/RHI/unit/rhi_unit_tests.exe
+
+# Test OpenGL backend
+RHI_BACKEND=OpenGL ./build/tests/RHI/unit/rhi_unit_tests.exe
+```