Trim UpdateDevice throw-test to the throw contract

The post-Disable frame + GetPlatformInfo assertion was demonstrating the canonical
UpdateDevice usage sequence, which is what TEST(Device, UpdateDevice) in BabylonJS#1687
does. The throw-test should focus on the throw contract only:
  - permitted pre-init
  - throws after EnableRendering
  - permitted again after DisableRendering

[Created by Copilot on behalf of @bghgary]

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: bghgary

Apps/UnitTests/Source/Tests.Device.D3D11.cpp

@@ -124,26 +124,19 @@ TEST(Device, UpdateDeviceThrowsWhenRenderingEnabled)
 
     Babylon::Graphics::Device device{config};
 
-    // Before EnableRendering: UpdateDevice should be permitted (init state has not been consumed
-    // yet). This pattern is used by callers who deferred their device choice.
+    // Pre-EnableRendering: UpdateDevice is permitted (init state has not been consumed yet).
+    // This pattern is used by callers who deferred their device choice.
     EXPECT_NO_THROW(device.UpdateDevice(deviceA.get()));
 
-    // After EnableRendering (driven by StartRenderingCurrentFrame): UpdateDevice must throw.
+    // After EnableRendering (driven by StartRenderingCurrentFrame): UpdateDevice throws.
     device.StartRenderingCurrentFrame();
     EXPECT_THROW(device.UpdateDevice(deviceB.get()), std::runtime_error);
     device.FinishRenderingCurrentFrame();
 
-    // Still initialized after the frame -- still throws.
+    // Still initialized between frames -- still throws.
     EXPECT_THROW(device.UpdateDevice(deviceB.get()), std::runtime_error);
 
-    // After DisableRendering, UpdateDevice is permitted again. The next frame picks up the new device.
+    // After DisableRendering: UpdateDevice is permitted again.
     device.DisableRendering();
     EXPECT_NO_THROW(device.UpdateDevice(deviceB.get()));
-
-    RenderTargetTexture rttB{CreateTestRenderTargetTexture(deviceB.get(), dimensions.cx, dimensions.cy)};
-    device.UpdateBackBuffer(rttB.View.get());
-    device.StartRenderingCurrentFrame();
-    device.FinishRenderingCurrentFrame();
-
-    EXPECT_EQ(device.GetPlatformInfo().Device, deviceB.get());
 }