Relay invokes its finalizer as soon as the provider stream ends, concurrently with
Hermes' consumer thread, so any finalizer that reads the consumer loop's closures
races the last chunk. #103104 moved `usage` onto the collector; the same race still
truncated the final tool-call `arguments` delta, dropped the last content delta, and
downgraded `finish_reason` to "stop" in Relay's annotated response (reproduced ~50%
of runs). Every other Relay integration (Anthropic, Bedrock, Codex) already
rebuilds from collector-observed chunks; this makes chat_completions match via
`_RelayChatAccumulator` and removes the single-field nonlocal stopgap.
Test: the ordering harness is shared, and a second invariant test pins the
tool-call/finish_reason case (fails on the stopgap, passes here). Hermes' own
returned response was never affected.
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