The chunked-request handler always returns a complete chat.completion
dict ("Streaming is not supported for chunked rotation requests"), but
stream=true callers still wrap it in the SSE stream generator, which
iterated the dict and emitted its KEYS as string chunks (data: "id",
data: "choices", ...). Clients crashed with "'str' object has no
attribute 'choices'". Previously masked: chunked requests died earlier
on the null-content TypeError, so this path was unreachable.

When the streaming wrapper receives a plain dict, emit one well-formed
chat.completion.chunk instead - delta carrying content and/or tool_calls
(with the per-entry index streaming requires), the real finish_reason,
usage attached - followed by data: [DONE].
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>