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

engine/src/RHI/OpenGL/OpenGLDevice.cpp

@@ -259,6 +259,13 @@ void OpenGLDevice::Shutdown() {
     m_initialized = false;
 }
 
+bool OpenGLDevice::MakeContextCurrent() {
+    if (m_hdc && m_hglrc) {
+        return ::wglMakeCurrent(m_hdc, m_hglrc) == TRUE;
+    }
+    return false;
+}
+
 void OpenGLDevice::SwapBuffers() {
     if (m_hdc) {
         ::SwapBuffers(m_hdc);