RHI: Replace IsFinalized/Finalize with IsValid/EnsureValid

Unified PSO validation semantics across D3D12 and OpenGL backends:
- IsValid() returns whether PSO is ready to use
- EnsureValid() ensures PSO is valid (compiles if needed)

Behavior by backend:
- D3D12: IsValid=false after creation, true after EnsureValid() with shaders
- OpenGL: IsValid always=true (immediate model)

Also added test_pipeline_state.cpp with 10 tests for RHIPipelineState.

tests/RHI/unit/CMakeLists.txt

@@ -13,6 +13,7 @@ set(TEST_SOURCES
     test_command_list.cpp
     test_command_queue.cpp
     test_shader.cpp
+    test_pipeline_state.cpp
     test_fence.cpp
     test_sampler.cpp
     ${CMAKE_SOURCE_DIR}/tests/opengl/package/src/glad.c