Stop seals the live bubble at the click and mutateStream drops every later
message.delta (queued-but-unflushed ones too), but the agent keeps streaming
until it honours the interrupt and persists every delta it delivered. The
interrupted message.complete carrying that persisted partial was ignored, so
state.db (and the next turn's context) held words the user never saw.
The interrupted completion now extends the stopped bubble to the persisted
partial, or adds a bubble when nothing had been painted. Extend-only: a
shorter or different text never replaces what was shown.
Refs #121594
4cdc58ad4c