[FEATURE]: Add macOS runner to CI test matrix

[brndnblck]

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Describe the enhancement you want to request

The current test matrix in .github/workflows/test.yml only covers Linux (blacksmith-4vcpu-ubuntu-2404) and Windows (blacksmith-4vcpu-windows-2025). There is no macOS runner.

The symlink regression in #16647 was macOS-specific — process.cwd() preserves symlink paths on macOS but Linux's getcwd() typically returns the canonical path. A macOS CI runner would have caught this before release.

Adding macOS to the test matrix would also catch:

• Symlink behavior differences — getcwd() semantics differ between Darwin and Linux
• Case-sensitivity differences — macOS HFS+/APFS is case-insensitive by default, Linux ext4 is case-sensitive
• Native binary/dylib differences — Bun native modules may behave differently
• Other POSIX divergences between Linux and Darwin

Related: #16647 (the symlink regression), #16651 (the fix), and this comment discussing cross-platform test coverage.