Add ExternalTexture MSAA render unit tests (#1712)

## Context

Coverage for MSAA + ExternalTexture rendering. Both tests pass on master
on D3D11 today; this PR characterizes what works so future device-loss /
`updateWrappedNativeTexture` MSAA work has a regression baseline.

## Changes

Two new gtest cases in `Tests.ExternalTexture.Msaa.cpp` that drive a
JS-side `RenderTargetTexture` whose `colorAttachment` is a
`wrapNativeTexture` over an MSAA GPU texture, then read back the pixels
(resolving MSAA before staging):

- `RenderWithMsaaSamples1` -- discriminator, asserts zero blend pixels
along the rotated red-plane edges so the samples=4 case is diagnostic.
- `RenderWithMsaaSamples4` -- asserts red/white blend pixels along the
same edges.

Cross-platform per the convention #1646 establishes: file lives in
shared `SOURCES`, gated by `SKIP_EXTERNAL_TEXTURE_TESTS` /
`SKIP_RENDER_TESTS`. CMake defines `SKIP_RENDER_TESTS` for D3D12 (no
impl).

Renames `Utils.{h,cpp,mm}` -> `Helpers.{h,cpp,mm}` under `namespace
Helpers`. Collapses `CreateTestTexture` and the new MSAA-RT helper into
one `Helpers::CreateTexture(d, w, h, arraySize=1, renderTarget=false,
samples=1)`. Adds `Helpers::ReadPixels` taking `PlatformInfo` so the
Metal path can use bgfx's command queue (avoids cross-queue sync).
Implemented on D3D11 and Metal; D3D12 / OpenGL throw (test skipped on
those configurations).

Will conflict with #1646 on `Helpers.h` / `Helpers.D3D11.cpp` /
`CMakeLists.txt` -- whichever lands first picks a winner.

[Created by Copilot on behalf of @bghgary]

---------

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

Author: bghgary

Apps/UnitTests/CMakeLists.txt

@@ -11,6 +11,7 @@ set(BABYLONJS_MATERIALS_ASSETS
     "../node_modules/babylonjs-materials/babylonjs.materials.js")
 
 set(TEST_ASSETS
+    "JavaScript/dist/tests.externalTexture.msaa.js"
     "JavaScript/dist/tests.javaScript.all.js"
     "JavaScript/dist/tests.shaderCache.basicScene.js"
     "JavaScript/dist/tests.shaders.cross.js")
@@ -22,12 +23,13 @@ set(SOURCES
     "Source/App.h"
     "Source/App.cpp"
     "Source/Tests.ExternalTexture.cpp"
+    "Source/Tests.ExternalTexture.Msaa.cpp"
     "Source/Tests.JavaScript.cpp"
     "Source/Tests.ShaderCache.cpp"
     "Source/Tests.ShaderCompilation.cpp"
     "Source/Tests.UniformPadding.cpp"
-    "Source/Utils.h"
-    "Source/Utils.${GRAPHICS_API}.${BABYLON_NATIVE_PLATFORM_IMPL_EXT}")
+    "Source/Helpers.h"
+    "Source/Helpers.${GRAPHICS_API}.${BABYLON_NATIVE_PLATFORM_IMPL_EXT}")
 
 if(GRAPHICS_API STREQUAL "D3D11")
     set(SOURCES ${SOURCES}
@@ -70,6 +72,10 @@ target_link_libraries(UnitTests
 
 target_compile_definitions(UnitTests PRIVATE ${ADDITIONAL_COMPILE_DEFINITIONS})
 
+if(GRAPHICS_API STREQUAL "D3D12")
+    target_compile_definitions(UnitTests PRIVATE SKIP_RENDER_TESTS)
+endif()
+
 add_test(NAME UnitTests COMMAND UnitTests)
 
 # See https://gitlab.kitware.com/cmake/cmake/-/issues/23543