OpenGL: Restructure tests similar to D3D12 layout

- Move old test files to new unit/integration structure
- Add OpenGL Test Fixture
- Update CMakeLists.txt for new layout
- Add OpenGL_Test_Restructuring_Plan.md

tests/RHI/OpenGL/unit/test_sampler.cpp

@@ -0,0 +1,60 @@
+#include "fixtures/OpenGLTestFixture.h"
+#include "XCEngine/RHI/OpenGL/OpenGLSampler.h"
+
+using namespace XCEngine::RHI;
+
+TEST_F(OpenGLTestFixture, Sampler_Initialize_Default) {
+    OpenGLSampler sampler;
+    OpenGLSamplerDesc desc;
+
+    bool result = sampler.Initialize(desc);
+
+    ASSERT_TRUE(result);
+    EXPECT_NE(sampler.GetID(), 0u);
+
+    sampler.Shutdown();
+}
+
+TEST_F(OpenGLTestFixture, Sampler_Initialize_Custom) {
+    OpenGLSampler sampler;
+    OpenGLSamplerDesc desc;
+    desc.minFilter = SamplerFilter::LinearMipmapLinear;
+    desc.magFilter = SamplerFilter::Linear;
+    desc.wrapS = SamplerWrapMode::Repeat;
+    desc.wrapT = SamplerWrapMode::Repeat;
+    desc.wrapR = SamplerWrapMode::ClampToEdge;
+
+    bool result = sampler.Initialize(desc);
+
+    ASSERT_TRUE(result);
+    EXPECT_NE(sampler.GetID(), 0u);
+
+    sampler.Shutdown();
+}
+
+TEST_F(OpenGLTestFixture, Sampler_Bind_Unbind) {
+    OpenGLSampler sampler;
+    OpenGLSamplerDesc desc;
+    sampler.Initialize(desc);
+
+    sampler.Bind(0);
+    GLint boundSampler = 0;
+    glGetIntegerv(GL_SAMPLER_BINDING, &boundSampler);
+    EXPECT_EQ(boundSampler, static_cast<GLint>(sampler.GetID()));
+
+    sampler.Unbind(0);
+    glGetIntegerv(GL_SAMPLER_BINDING, &boundSampler);
+    EXPECT_EQ(boundSampler, 0);
+
+    sampler.Shutdown();
+}
+
+TEST_F(OpenGLTestFixture, Sampler_GetID_ReturnsValid) {
+    OpenGLSampler sampler;
+    OpenGLSamplerDesc desc;
+    sampler.Initialize(desc);
+
+    EXPECT_NE(sampler.GetID(), 0u);
+
+    sampler.Shutdown();
+}