The no-tool native stream applied _decode_escaped_newlines_outside_fences
to each streamed chunk independently, but a fenced code block almost never
opens and closes within one token-sized chunk, so the per-chunk fence regex
never saw a complete ``` pair and decoded every literal \n inside the fence.
Code like print("a\nb") reached the streamed answer and the persisted
message content with a real newline, while the non-streaming path and the
tool-mode simulated stream preserved it.
Decode the native stream with a fence-stateful decoder instead: it carries
whether the stream is inside a fence across chunks and holds back a
trailing partial ```/`\n` sequence, so the joined stream equals the one-shot
decode of the same reply. A fence left unclosed by a truncated reply keeps
its escapes, matching how an unterminated fence still renders as code.
Document native no-tool streaming in the MindIE decode contract.
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