Fix Metal tests to SKIP instead of FAIL when Apple Silicon GPU is unavailable

[Copilot]

14 Metal tests were reporting FAILED on the macOS CI runner (which lacks Apple Silicon) instead of SKIPPED, because they used ASSERT_NE(renderer, nullptr) rather than GTEST_SKIP() after a failed renderer construction.

Changes

• tests/RendererParityTests.cpp / tests/ComputeTests.cpp: Replace ASSERT_NE ( renderer, nullptr ) with the established skip pattern:

  if ( renderer == nullptr )
  {
      GTEST_SKIP() << "Metal renderer unavailable on this host.";
  }

• tests/RendererParityTests.cpp (shadow/environment tests): Switch from ConstructRenderer(...) — which throws on unavailable backends, causing an uncaught-exception failure — to TryConstructRenderer(...), which returns nullptr and lets the skip guard above fire cleanly.
• tests/RenderTestWindow.h: Add a TryConstructRenderer overload accepting RendererSettings to support the above.

[Kwizatz]

@copilot Please fix ALL CI builds, looks like we have 3 failures.

[Kwizatz]

There are 3 other builds failing, none should fail.

[Kwizatz]

Merged