ci(dry-run): stop scarce Intel-macOS runner from gating smoke/soak

GitHub is sunsetting Intel-macOS runners, so the macos-15-intel build leg is
repeatedly cancelled before it can be allocated, turning the aggregate build
result non-success and skipping the smoke + soak jobs even though every other
build/test passed.

Mark only the macos-15-intel matrix leg continue-on-error, and relax the
smoke/soak gate to run unless builds were skipped or a leg genuinely failed
(tolerating a cancelled aggregate caused solely by the non-blocking Intel leg).
All other platforms remain required.