#3891 extension_tarball_analyzer_test fails on macOS: tar --mode is GNU-only

tests/infrastructure/extension_tarball_analyzer_test.ts builds its unreadable-doc fixture with: tar rf --mode=0000 -C . --mode is a GNU tar option; macOS ships bsdtar, which fails with: tar: Option --mode=0000 is not supported. So the test 'analyzer - an unreadable doc marks the secret-docs scan truncated' (line 1081, added in 0d66d246 / PR 1405) fails on every macOS host. It passes on Linux CI, but local verify-build runs on the host, so every PR verified from a Mac fails run-tests and cannot reach gate.allPassed. Fix: build the fixture portably, e.g. chmod the files to 0000 before appending (bsdtar and GNU tar both record the on-disk mode), or write the tar headers in Deno instead of shelling out to tar.