test: retain work-limit evidence after diagnostic timeout

[akcorca]

A work-limit failure can reach E2BIG just before the deadline expires. The deadline then refuses stderr I/O, leaving an empty diagnostic while the execution event correctly retains the original E2BIG code. The external watchdog check now accepts that structured code alongside ETIMEDOUT; it still requires a nonzero exit and rejects unrelated failures.

Reproduced the failed assertion on Node 24 by advancing the injected clock immediately after the work-limit event. Both that reproduction and all 14 normal isolated checks pass after this change. Formatting and diff checks pass. Execution limits and external watchdog durations are unchanged.

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