Crawler unit tests read the developer's real HOME, YARN_NODE_LINKER and VIRTUAL_ENV: 12 pkg_managers/pnpm_layout tests fail under a user nodeLinker setting #1919

[agent] Filed by the scheduled architecture audit (audit-core). Register: A48.

Kind: bug (test hermeticity). Register ID: A48.

Problem

The PnP detection tests in crawlers/pkg_managers.rs go through YarnEnv::for_project, which reads the process env and ~/.yarnrc.yml (pkg_managers.rs L452-L461 and the YarnEnv resolution above it), instead of the injected bare_env the module already offers. pnpm_layout::modules_dir_setting also walks the tempdir's ancestors (e.g. /tmp/.npmrc).

Executed on a build of main @ cbbbdb9 (by the audit, nothing committed):

• HOME pointing at a dir whose .yarnrc.yml says nodeLinker: node-modules → 10 pkg_managers tests fail, plus pnpm_layout::a_configured_modules_dir_without_a_store_is_not_pnpm;
• YARN_NODE_LINKER=node-modules → 12 tests fail.

By code reading (unexecuted), the python crawler's test_crawl_all_python, test_crawl_all_recovers_metadata_less_package, test_crawl_all_not_poisoned_by_body_headers, base_copy_behind_a_system_site_venv_is_crawled and venv_copy_hides_the_base_copy_of_the_same_package go through PythonCrawler::get_site_packages_paths (L3573),``` which returns early on a real VIRTUAL_ENV (L752-L760),```` so they should fail in an activated shell.

CI passes only because its env is clean; a contributor with a global Yarn nodeLinker or an active venv sees red tests on a clean checkout.

Proposed change

• Drive the PnP tests through YarnEnv/bare_env (or live_pnp_marker_with) instead of for_project; give pnpm_layout tests an injected .npmrc chain or a hermetic root.
• Drive the five python tests through the _with env-injected variants (find_local_venv_site_packages_with + env_of).
• Where a test must read the process env, use the shared guard from Share one scoped env-var test guard: EnvGuard is copy-pasted 11 times across core unit and integration tests #1889 to clear HOME, YARN_*, VIRTUAL_ENV, CONDA_PREFIX.

Size

3 files, test-only, ~+40/-20.

Acceptance criteria

• cargo test -p socket-patch-core --lib crawlers:: passes with YARN_NODE_LINKER=node-modules, with a HOME/.yarnrc.yml setting nodeLinker: node-modules, and with VIRTUAL_ENV set to a real venv.
• No production behavior change.