Copy test fixture trees through one tests/common::copy_tree: 38 recursive copiers, 20 identical #1979

[agent] Filed by the scheduled architecture audit (audit-cli-tests). Register: register.

Kind: refactor. Register ID: T64.

Problem

crates/socket-patch-cli/tests has 38 recursive directory copiers: 26 copy_dir_recursive, 12 copy_tree and a few copy_dir. tests/common has none, although 129 suites include it.

• Identical copies. 20 bodies are identical after whitespace normalization, for example e2e_redirect_cargo_build.rs L139,`` e2e_vendor_golang_build.rs L220 and mode_migration_npm.rs L120.```` The full list:
  • e2e_redirect_{bun,cargo,pnpm,yarn_berry,yarn_classic}_build
  • e2e_vendor_{bun,cargo,gem,golang,pnpm,pypi,yarn_berry,yarn_classic}_build
  • e2e_vendor_yarn_classic_dev_flow
  • e2e_vendored_production
  • e2e_yarn4_{pnpm_linker,workspaces}_build
  • mode_migration_{bun,cargo,npm}
• Already "shared" copies. Three crate-local modules export the same body as pub: composer_e2e_common L323,````` maven_build_common L509 and `npm_e2e_common/manifestless.rs` L30. `prebuilt_common` keeps a private copy at L722,`````` and `e2e_nuget_dotnet_build.rs` L320 re-copies it.
• Drifted copies. The other 15 have drifted:
  • e2e_redirect_gem_build.rs L253 silently accepts a missing source.
  • Some copies take a skip list: copy_tree(from, to, skip) ×2.
  • e2e_gradle_agent_build.rs L1077 walks with walkdir.
  • e2e_embedded_vex.rs L279 copies one level only.

Same family, also to collapse once the helper exists:

• Scan proxy. start_proxy and assert_proxy_served_scans are byte-identical in e2e_cargo.rs L33/L112, e2e_composer.rs L31/L79, e2e_maven.rs L33/L91 and e2e_nuget.rs L33/L162. e2e_gem.rs L292 is a deliberate by-package variant, but the composer doc still says that suite has the same shape.
• Hosted pins. hosted_pin_count is written ×8 (e.g. e2e_hosted_production.rs L421, e2e_bun_lockb.rs L125, in_process_vendor.rs L290) and hosted_pinned ×14, beside common::envelope::hosted_pins.

Proposed change

1. Add pub fn copy_tree(from: &Path, to: &Path) to tests/common/mod.rs, using the identical body above.
2. Delete the 20 identical copies, the 4 *_common copies and the nuget copy. Import from common instead.
3. For the drifted callers:
   • Keep a skip-list or missing-source variant only where a caller needs it, as copy_tree_except(from, to, skip) in the same module.
   • Make the gem caller check existence itself.
4. Optional follow-up in the same PR or the next one:
   • Move the 4 identical start_proxy/assert_proxy_served_scans into a tests/common/scan_proxy.rs.
   • Replace hosted_pin_count with envelope::hosted_pins(..).len().

Size

About 40 test files. Roughly −450 lines.

Acceptance criteria

• No test changes its assertions. The touched suites build and pass: cargo test -p socket-patch-cli --tests --no-run, plus the host-runnable ones that use the copies (mode_migration_*, e2e_redirect_cargo_build).
• grep -rnE "fn (copy_tree|copy_dir_recursive)\(" crates/socket-patch-cli/tests shows only common/.