Isolate run-all default-concurrency tests from ambient env

[TheGreatAxios]

Summary

scripts/run-all.test.ts runProbe spread process.env into the child, so the default-sequential assertion (peakOverlap == 1) failed when the parent had WORKBENCH_CHECK_CONCURRENCY set (for example while running the gate itself). Tests that intend an override already inject the var; the default path now clears it unless a test sets it. A new test sets the ambient var and still expects sequential default.

CL-7240

Test plan

• bun test scripts/run-all.test.ts (15 pass, including the new ambient-env isolation test and existing override tests)
• Red: ambient WORKBENCH_CHECK_CONCURRENCY=4 made the new test fail with peakOverlap 4 vs 1
• Green after runProbe stopped inheriting the var unless extraEnv sets it

Fixes CL-7240

[TheGreatAxios]

Fixes CL-7240

[TheGreatAxios]

Self-review

Verdict: minimal, correctly targeted fix — runProbe no longer leaks an ambient WORKBENCH_CHECK_CONCURRENCY into default-concurrency probes, and the new test proves it with a real red/green cycle. No defects found.

• CONFIRMED-OPEN: none
• DISMISSED: checked for exactOptionalPropertyTypes hazards, bare catches, as T on untrusted input, fallbacks left beside new paths, and product logic leaking into apps/* — none apply, this is test-harness-only. The Object.hasOwn(extraEnv, ...) check correctly distinguishes "override intentionally set" from "not set", including the WORKBENCH_CHECK_CONCURRENCY: "0" rejection test.
• Verified the fix actually addresses the stated bug: ran the full suite locally with WORKBENCH_CHECK_CONCURRENCY=4 set on the parent process before either test existed in isolation — passes with the fix, and the PR description documents the red state without it.

Targeted tests run:

• bun test scripts/run-all.test.ts — 15 pass, 0 fail
• WORKBENCH_CHECK_CONCURRENCY=4 bun test scripts/run-all.test.ts — 15 pass, 0 fail (ambient-leak repro confirms the fix holds)

[linear-code]

CL-7240