RHI: Internalize OpenGL-specific methods in OpenGLDevice

- Remove GetTextureUnitAllocator/GetUniformBufferManager from public interface
- Remove SwapBuffers from public interface (friend OpenGLSwapChain can still access)
- Add MakeContextCurrent() and GetNativeContext() public methods
- Update integration tests to use MakeContextCurrent() instead of wglMakeCurrent
- Update RenderDoc calls to use GetNativeContext() instead of GetGLContext()
- Remove Device_SwapBuffers_NoErrors test (SwapBuffers no longer public)
- Mark Priority 7 as completed in RHI_Design_Issues.md

tests/RHI/OpenGL/unit/test_device.cpp

@@ -22,14 +22,4 @@ TEST_F(OpenGLTestFixture, Device_GetDeviceInfo_ReturnsValid) {
     EXPECT_FALSE(info.renderer.empty());
     EXPECT_GE(info.majorVersion, 3);
     EXPECT_GE(info.minorVersion, 0);
-}
-
-TEST_F(OpenGLTestFixture, Device_SwapBuffers_NoErrors) {
-    OpenGLDevice device;
-    device.InitializeWithExistingWindow(GetWindow());
-
-    device.SwapBuffers();
-
-    GLenum error = glGetError();
-    EXPECT_EQ(error, GL_NO_ERROR);
 }