This commit addresses the issue of relying on `compose_state` for
retrieving the `stream_id` to display warning banners. Previously,
warnings were shown for syntax in the message edit box based on
whether that syntax would trigger a warning for the draft content (if
any) currently in the compose box.
We fix this by using a new `get_stream_id_for_textarea` function to
obtain the correct `stream_id` value for the check being done.
Fixes: #25410.