Tracking: share CLI test helpers through one test-support module instead of 100+ per-file copies

[mikolalysenko]

[agent] Filed by the scheduled architecture audit routine (CLI and core). Register: register comment.

Kind: tracking. Source: review Part 6.4 and Part 8.5 D; register C30.

Problem (re-counted on 045d7ec)

The CLI already has a shared tests/common/mod.rs (every test target can reach it through #[path = "../common/mod.rs"]), but most files keep their own copies:

• fn binary() in 103 files, 7 variants (PathBuf, &'static str, one env-overridable); common::binary exists at L35-L37.
• fn git_sha256 in 86 files, 10 variants, although common::git_sha256 exists and core exports compute_git_sha256_from_bytes.
• 15 scrub_socket_env copies with 14 bodies, and 10 files that spawn the binary with no scrub at all. This one is a correctness problem, not just duplication: on 045d7ec an ambient SOCKET_DRY_RUN=true turns 18 of the 19 unscrubbed repair_vendor_e2e tests red while every scrubbed test in the same target is unaffected (details in Spawn every CLI test child through one hermetic Command builder; 10 test files inherit ambient SOCKET_* today #823).
• xorshift64* is written four times in core (test_rng.rs, oracle_support.rs, npm_crawler/oracle.rs, patch/copy_tree.rs); test_rng is the intended shared one.
• The VEX e2e helpers are split across vex_e2e_common, vex_pdm_hatch_common, vex_pipenv_pip_common, vex_pypi_real_common and common/yarn_classic_vex.rs.

Target design

One test-support surface: tests/common for the CLI (it already reaches every target), which can later become a socket-patch-test-support dev crate if core and CLI tests need to share it. It owns the hermetic Command builder, binary(), git_sha256 (delegating to core), the envelope parsers and the seeded RNG. Each child deletes the copies it replaces, and none touches production code.

Children (in order)

• Spawn every CLI test child through one hermetic Command builder; 10 test files inherit ambient SOCKET_* today #823 Spawn every CLI test child through one hermetic Command builder; delete the 15 scrub_socket_env copies and cover the 10 unscrubbed files.
• Replace the 103 private binary() with common::binary (mechanical; e2e_redirect_pnpm_build.rs keeps its env override as a named helper).
• Replace the 86 private git_sha256 with common::git_sha256, built on compute_git_sha256_from_bytes.
• Route core's three private xorshift copies through test_rng.
• Merge the VEX e2e helper modules.

Only the first child is filed; the rest are mechanical and can be filed as the first lands.

Acceptance criteria

• Each child lands as its own PR, with all tests green.
• At the end, grep -rn "fn binary()\|fn git_sha256\|fn scrub_socket_env" crates/socket-patch-cli/tests matches only tests/common.

Dependencies

Unblocked. C31 (fewer test executables) and #793 (RunCtx, removing #[serial]) are easier after it.

[mikolalysenko]

[agent] Triaged: priority:p3 (tracking issue for a test-only refactor). Not a duplicate; first child is #823.

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