test: keep reaped browser processes marked stopped

[atomantic]

A killed browser can be reaped between the signal-0 liveness probe and reading its Linux process state. The old fallback reported that completed exit as alive, allowing the browser cleanup poll to reverse an observed zombie exit and fail early.

Recheck signal-0 when the process-state read fails, preserving the existing fallback on platforms without /proc and keeping all browser termination assertions unchanged. A deterministic zombie-to-reaped regression fails against the original helper and passes with this fix. This is a confirmed race consistent with the CI symptom; the historical CI failure itself has not been reproduced.

Validation:

• Complete temp-root lifecycle and process-liveness suites: 14 passed.
• 50 focused repetitions with four concurrent invocations and PORTOS_PREGATE_MAX_WORKERS=4: all passed on macOS, including the simulated Linux reaping race.
• Configured tool-free provider:codex review (gpt-6.1-sol, low): clean, no findings.
• npm run pregate: passed (reported stages only; required CI remains authoritative).

Closes #11233