Sub508 log: t2_n_2 → HTTP 500 → engine crash → ALL subsequent 24+ tests HTTP 500.
The scheduler config call may throw if engine is in a bad state. Wrapping in
try-catch ensures we return 400 (not 500) and the engine stays alive.
When n>=2, all_previous_token_ids entries pointed to the SAME list,
so appending tokens for choice 0 corrupted choice 1's history.
Same for tool_parsers: all choices shared one stateful parser instance.
Changed to list comprehensions that create independent objects.
Found via CCCL result_policy.cuh read: distributed result delivery
requires isolated per-rank state — same principle applies to
per-choice token tracking in vLLM streaming.