Share one scoped env-var test guard: EnvGuard is copy-pasted 11 times across core unit and integration tests #1889

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

Kind: refactor (tests). Register ID: A37.

Problem

Tests that mutate process env each define their own restore-on-drop guard.

These three copies are byte-identical:

• go_crawler.rs L1518-L1542
• deno_crawler.rs L945-L969
• maven_crawler.rs L2508-L2532

These are variants:

• npm_crawler.rs L5880, which takes an OsStr value
• python_crawler.rs L7878 and L8107
• vendor/maven_repo.rs L2649
• patch/jvm_jar.rs L837
• tests/upstream_restore_golden.rs L188
• tests/crawler_deno_e2e.rs L24
• tests/crawler_npm_e2e.rs L357

There are also closure-style twins: utils/socket_cli_config.rs with_env (L311) and utils/fs.rs with_home_env (L1489).

grep -rn "struct EnvGuard" crates finds 11 definitions.

Proposed change

• Add one #[cfg(test)] pub(crate) mod test_env in core, beside test_rng. It holds EnvGuard::set(key, value), ::unset(key) and a multi-key form taking OsStr values, plus with_env(pairs, f).
• Unit tests use it.
• The three integration-test files share one copy in tests/common/, or through a test-support feature if one already exists.
• Where a function under test already takes injected env (*_with variants), switch the test to that and drop the guard.

Size

About 12 files, about −150 lines. Tests only.

Acceptance criteria

• No production code changes.
• Every touched test still runs under #[serial] where it did before and passes.
• grep -rn "struct EnvGuard" crates/socket-patch-core/src finds one definition.