update_chat_by_id re-derived assistant content from `output` on every
save (serialize_output) so frontend edits to output items reflect in
content. But it ran unconditionally, so content set independently of
output — an __event_emitter__ {"type":"replace"} from an Action, or an
outlet filter footer — was reverted to the original output-derived text
on the next save. The reload reads chat.chat directly, so the change
vanished after navigating away (regression vs 0.9.2, which predates the
output mechanism).
Re-derive only when the message's `output` actually changed versus what's
stored, which still reflects genuine output edits but leaves
independently-set content intact.
Fixes#24585
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
