When a streaming edit fails mid-stream (flood control, transport error)
and a tool boundary arrives before the fallback threshold is reached,
the pre-boundary tail in `_accumulated` was silently discarded by
`_reset_segment_state`. The user saw a frozen partial message and
missing words on the other side of the tool call.
Flush the undelivered tail as a continuation message before the reset,
computed relative to the last successfully-delivered prefix so we don't
duplicate content the user already saw.
1d1e1277e4