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/CMakeLists.txt

@@ -43,6 +43,7 @@ add_subdirectory(RHI)
 add_subdirectory(RHI/D3D12)
 add_subdirectory(RHI/OpenGL)
 add_subdirectory(Resources)
+add_subdirectory(Input)
 
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 # Test Summary