feat(opencode): add macOS to test matrix

[brndnblck]

Issue for this PR

Closes #16661
Related: #16647

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds a macos-latest runner to both the unit and e2e test matrices in .github/workflows/test.yml.

The test matrix currently only covers Linux and Windows. The symlink regression in #16647 was macOS-specific — process.cwd() preserves symlink paths on macOS but Linux's getcwd() returns the canonical path — and would have been caught by a macOS CI runner.

This also improves coverage for case-sensitivity differences (macOS APFS is case-insensitive by default) and other Darwin/Linux POSIX divergences.

Uses macos-latest, consistent with the runner label already used in publish.yml.

How did you verify your code works?

• Verified setup-bun action already handles macOS (case on line 21)
• Confirmed macos-latest is the same runner label used in other workflows (publish.yml)
• Playwright install uses bunx playwright install (no --with-deps), matching the Windows pattern

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Hey! Your PR title ci: add macOS to test matrix doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[brndnblck]

Note: MacOS tests pass here because the issue from #16647 also lacked test coverage. Both the test coverage that was needed for MacOS and the fix exist in #16651.

This PR will ensure MacOS gets included in the test matrix going forward, and we don't regress.