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.

engine/src/RHI/D3D12/D3D12PipelineState.cpp

@@ -55,7 +55,8 @@ bool D3D12PipelineState::Initialize(ID3D12Device* device, const D3D12_GRAPHICS_P
         m_inputElements.push_back(desc.InputLayout.pInputElementDescs[i]);
     }
     
-    return Finalize();
+    EnsureValid();
+    return m_finalized;
 }
 
 D3D12PipelineState::~D3D12PipelineState() {
@@ -130,12 +131,13 @@ void D3D12PipelineState::SetComputeShaderBytecodes(const D3D12_SHADER_BYTECODE&
     m_csBytecode = cs;
 }
 
-bool D3D12PipelineState::Finalize() {
-    if (m_finalized) return true;
+void D3D12PipelineState::EnsureValid() {
+    if (m_finalized) return;
     if (HasComputeShader()) {
-        return CreateD3D12ComputePSO();
+        CreateD3D12ComputePSO();
+    } else {
+        CreateD3D12PSO();
     }
-    return CreateD3D12PSO();
 }
 
 bool D3D12PipelineState::CreateD3D12PSO() {