Models that interleave their reasoning_content and content token streams
(Kimi/DeepSeek/GLM-style routes) emit text -> reasoning -> text deltas
within a single tool-bounded segment. Appending each delta as its own
part shredded one sentence into "Let me" / Thinking / "verify the file",
with a Thinking disclosure wedged mid-sentence.
Coalesce streaming deltas into the most recent same-type part within the
current segment (bounded by any non-streaming part, e.g. a tool call).
The opposite streaming channel is transparent, so a reasoning burst
between two content deltas no longer opens a fresh text part, while a
real tool call still starts a new segment and preserves narration order.
Data-layer only; the renderer already groups consecutive reasoning.
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