test(dlx): cover spawn.mts to 100%

Adds unit tests for validatePackageName, spawnDlx, spawnToolVfs, and
downloadGitHubReleaseBinary covering TOCTOU lock handling (alive/dead/
invalid PID branches, gone lock file), zip-slip protection, symlink-
escape detection, tar.gz extraction, unsupported archive rejection, and
error wrapping.

Author: jdalton