fix: resolve environment test quality findings

[mldangelo-oai]

Summary

• simplify the Pop!_OS distro-detection test so it reads the real temp fixture directly
• remove the duplicate fixture payload and the misleading unused Path shim
• delete the dead local helper flagged by CodeQL in the /usr/lib/os-release fallback test

Why

GitHub Security and quality currently reports two AI findings plus one standard CodeQL maintainability finding in tests/test_environment.py. These are test-only cleanup issues, so this PR resolves them without changing runtime behavior.

Validation

• uv run pytest tests/test_environment.py
• uv run pytest -m 'not smoke'
• uv run ruff check tests/test_environment.py
• git diff --check