test: stabilize env boot validation

[birkskyum]

Summary

• reuse the readiness response when asserting the expected boot-validation 500
• wait for each preview process to exit before starting the next one
• avoid the socket-close race seen in the failed release run

No changeset is needed because this only changes the test harness.

Testing

• Start env suite: 47/47
• production env path repeated five times: 17/17 each run

[changeset-bot]

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Latest commit: 1de3195

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commit: 1de3195