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/test_shader.cpp

@@ -0,0 +1,123 @@
+#include "fixtures/RHITestFixture.h"
+#include "XCEngine/RHI/RHIShader.h"
+
+using namespace XCEngine::RHI;
+
+TEST_F(RHITestFixture, Shader_Compile_FromSource) {
+    const char* vertexShader = R"(
+        cbuffer ConstantBuffer : register(b0)
+        {
+            float4x4 worldViewProj;
+        }
+        struct VS_INPUT {
+            float3 pos : POSITION;
+        };
+        struct VS_OUTPUT {
+            float4 pos : SV_POSITION;
+        };
+        VS_OUTPUT main(VS_INPUT input) {
+            VS_OUTPUT output;
+            output.pos = float4(input.pos, 1.0f);
+            return output;
+        }
+    )";
+
+    RHIShader* shader = GetDevice()->CompileShader({});
+    ASSERT_NE(shader, nullptr);
+
+    shader->Shutdown();
+    delete shader;
+}
+
+TEST_F(RHITestFixture, Shader_GetType) {
+    RHIShader* shader = GetDevice()->CompileShader({});
+    ASSERT_NE(shader, nullptr);
+
+    ShaderType type = shader->GetType();
+    EXPECT_GE(static_cast<uint8_t>(type), static_cast<uint8_t>(ShaderType::Vertex));
+
+    shader->Shutdown();
+    delete shader;
+}
+
+TEST_F(RHITestFixture, Shader_IsValid) {
+    RHIShader* shader = GetDevice()->CompileShader({});
+    ASSERT_NE(shader, nullptr);
+
+    EXPECT_TRUE(shader->IsValid());
+
+    shader->Shutdown();
+    delete shader;
+}
+
+TEST_F(RHITestFixture, Shader_Bind_Unbind) {
+    RHIShader* shader = GetDevice()->CompileShader({});
+    ASSERT_NE(shader, nullptr);
+
+    shader->Bind();
+    shader->Unbind();
+
+    shader->Shutdown();
+    delete shader;
+}
+
+TEST_F(RHITestFixture, Shader_SetInt) {
+    RHIShader* shader = GetDevice()->CompileShader({});
+    ASSERT_NE(shader, nullptr);
+
+    shader->SetInt("testInt", 42);
+
+    shader->Shutdown();
+    delete shader;
+}
+
+TEST_F(RHITestFixture, Shader_SetFloat) {
+    RHIShader* shader = GetDevice()->CompileShader({});
+    ASSERT_NE(shader, nullptr);
+
+    shader->SetFloat("testFloat", 3.14f);
+
+    shader->Shutdown();
+    delete shader;
+}
+
+TEST_F(RHITestFixture, Shader_SetVec3) {
+    RHIShader* shader = GetDevice()->CompileShader({});
+    ASSERT_NE(shader, nullptr);
+
+    shader->SetVec3("testVec3", 1.0f, 2.0f, 3.0f);
+
+    shader->Shutdown();
+    delete shader;
+}
+
+TEST_F(RHITestFixture, Shader_SetVec4) {
+    RHIShader* shader = GetDevice()->CompileShader({});
+    ASSERT_NE(shader, nullptr);
+
+    shader->SetVec4("testVec4", 1.0f, 2.0f, 3.0f, 4.0f);
+
+    shader->Shutdown();
+    delete shader;
+}
+
+TEST_F(RHITestFixture, Shader_SetMat4) {
+    RHIShader* shader = GetDevice()->CompileShader({});
+    ASSERT_NE(shader, nullptr);
+
+    float mat[16] = {};
+    shader->SetMat4("testMat4", mat);
+
+    shader->Shutdown();
+    delete shader;
+}
+
+TEST_F(RHITestFixture, Shader_GetNativeHandle) {
+    RHIShader* shader = GetDevice()->CompileShader({});
+    ASSERT_NE(shader, nullptr);
+
+    EXPECT_NE(shader->GetNativeHandle(), nullptr);
+
+    shader->Shutdown();
+    delete shader;
+}