A new assistant message was seeded with the previous message's
reasoningText (and isReasoning flag) via onMessageStart, and
resetStreaming() never cleared the shared streaming state. If the
next reply produced no reasoning, the stale trace stayed glued to
the new bubble and got persisted to Room.
- onMessageStart: new message + StreamingState start with empty
reasoning instead of inheriting the previous turn's
- resetStreaming: clears isReasoning + reasoningText on the shared
state (finalized messages keep their own copy for the card)
- regression tests: MessageStart seeding, no-reasoning persistence,
reset clearing, finalized-message preservation
Co-authored-by: M57 <hy4ri@users.noreply.github.com>
