ci: promote windows-latest + macos-latest from soft gates to required

[HumanBean17]

Context

PR #371 added a 3-OS CI matrix. ubuntu-latest is the required merge gate; macos-latest + windows-latest were added as continue-on-error: \${{ matrix.os != 'ubuntu-latest' }} (non-blocking) because the native kuzu/ladybug graph layer had never been exercised on those OSes. Both have now gone green.

Action

Once each has been green across several merges, promote it to a hard gate by dropping it from the continue-on-error expression in .github/workflows/test.yml:

continue-on-error: ${{ matrix.os != 'ubuntu-latest' }}   # remove an OS here to make it required

Acceptance

• macOS and/or Windows legs are required (a deliberately broken commit fails the PR).
• fail-fast: false retained so one OS failing doesn't cancel the others.