test/apps-idle.test.ts: production preview reads "failed" once in seven runs #1174

test/apps-idle.test.ts moves from EXCLUDED to GATE in scripts/gate.ts. I couldn't run it here, so the seven-run stability proof is not done. The sandbox refuses the unix-socket listen the test's server needs (EPERM).

• The race: the cause is in the test. Production deploys first and alone under the deploy gate, so on a slow machine it can still be coming up when the previews are already running. The test checked production straight away and, when it had failed, said nothing about why.
• The fix: it already landed on main in bb67382. The test now waits for production to leave "deploying" and, if it isn't running, fails with production's own error text. I found no race in the runtime, so I changed nothing in src/.
• Checked: bun test test/check-list.test.ts passes (8 tests) with the file in the gate.
• Not checked: please run bun test test/apps-idle.test.ts --timeout 60000 a few times, or let the forge check do it, before merging. If production still reads "failed", the error text now in the assertion will show the real cause.

Closes #1055