Fix macOS test fixtures for paths and disk rollback

[ESultanik]

The macOS unit suite now avoids creating an invalid UTF-8 filename on APFS and deterministically tests Lima's YAML rollback when truncate cannot start. The rollback fixture runs in an isolated child with a controlled executable search path, so it neither finds a host truncate nor changes sibling tests' environment. Linux retains the filesystem-backed invalid-path test.

Closes #508. Closes #509.

Validation: before rebasing, macOS workspace tests, workspace Clippy with -D warnings, formatting, and all pre-commit checks passed. After rebasing onto current main, Linux workspace tests, workspace Clippy with -D warnings, formatting, TOML formatting, dependency checks, and all applicable pre-commit hooks pass. Removing the spawn-error rollback in the rebased tree deliberately makes the repaired test fail on the changed YAML; restoring it makes the test pass. The full rebased diff passed closeout review. Production behavior is unchanged. macOS tests were not repeated after the rebase; fresh Linux CI and all CodeQL analyses passed on the rebased head.

[ESultanik]

Merge plan for this four-PR stack:

Stack order: #512 → #513 → #514 → #516. Each PR should ultimately merge into main, in that order; do not merge a follow-on PR into its current feature-branch base.

After a predecessor merges, retarget the next PR to main (verify the base even if GitHub updates it automatically). Integrate main with a merge commit if needed, resolve conflicts, and check that the final diff contains only that PR's intended change. Preserve published history: no force-push/rebase is needed. Refresh required CI and review; repeat validation affected by any changes to the tested tree.

This PR is the first step and is already ready for review. It changes test fixtures, not guest behavior. macOS workspace tests and the deliberate rollback regression check passed; Linux CI and CodeQL are green. Obtain the usual maintainer approval and confirm required checks remain green, then merge #512 into main. This PR does not need to wait for the feature PRs' outstanding Firecracker runs.

The next action is to retarget #513 from fix/macos-test-fixtures to main and complete its platform gate. #514 follows #513, and #516 follows #514. The three drafts are intentional: their remaining platform gate is Firecracker VM integration. Each has a separate comment with its command and readiness checklist.

#515 is an independent follow-up for existing postStartCommand array semantics and is not a prerequisite for merging this stack.

[hbrodin]

Reviewed the full rebased diff at 6a13b64 using current main guidance. No actionable findings remain. Linux workspace tests, workspace Clippy, formatting, dependency checks, and applicable pre-commit hooks pass. Deliberately removing the spawn-error rollback makes the repaired test fail; restoring it passes. macOS tests were reported before the rebase and were not repeated afterward. Production behavior is unchanged; fresh CI must pass before merge.