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/integration/sphere/main.cpp

@@ -174,7 +174,7 @@ int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine
         return -1;
     }
 
-    RenderDocCapture::Get().SetDevice(device.GetGLContext());
+    RenderDocCapture::Get().SetDevice(device.GetNativeContext());
 
     ShowWindow(hwnd, nShowCmd);
     UpdateWindow(hwnd);
@@ -312,7 +312,7 @@ int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine
             TranslateMessage(&msg);
             DispatchMessageW(&msg);
         } else {
-            wglMakeCurrent(device.GetPresentationDC(), device.GetGLContext());
+            device.MakeContextCurrent();
 
             renderCount++;
             GLenum err = glGetError();