test(dev): cover linux lab prerequisite paths

[codeforester]

Summary

• Cover Multipass discovery, timeout, nonzero stdout/stderr diagnostics, and successful version parsing.
• Exercise Linux-lab setup when Multipass is already installed, Homebrew is absent, delegated installation succeeds, or installation raises ArtifactError.
• Assert stable BASE-D108, exit codes, messages, and actionable fixes without installing Multipass or creating VMs.

linux_lab.py now has 100% statement and branch coverage in its focused suite (issue targets: 85%/75%).

Issue

Fixes #2051

Validation

• Focused Linux-lab suite (12 passed, 6 subtests; 100% statements and branches)
• Pylint on the touched test file
• Full Python coverage suite and ratchet (1,115 passed, 327 subtests; 90.24% statements, 80.26% branches, 87.79% combined)
• ./bin/base-test with supported sibling source overrides (1,115 Python tests, 327 subtests, and 899 BATS tests passed)

Demo Impact

None. Test-only hardening; no user-facing behavior changes.

Notes

All executable lookup, subprocess, and Homebrew behavior is mocked. Documentation and AI context are not applicable because production behavior is unchanged.

Checklist

• Branch name follows <category>/<issue>-<YYYYMMDD>-<slug>, and the prefix matches the issue's single standard category label.
• PR is scoped to one issue, unless a documented multi-issue exception applies.
• PR body explains what changed and how it was validated.
• Validation commands were run from the current checkout or worktree, or unavailable checks are explained.
• Relevant BATS and Python tests pass.
• Bug fixes include regression proof or a documented reproduction when practical.
• Documentation is updated when behavior or user-facing commands change.
• AI context is updated in .ai-context/, or the PR body explains why it is not applicable.
• PR includes Fixes #<issue> or Closes #<issue> when it should close the issue.
• Demo Impact is meaningful for needs-demo work, or explicitly says None.