When streaming is enabled, text chunks are sent to the user in
real-time including raw MEDIA: tags. The normal post-processing in
_process_message_background is skipped when already_sent=True, so
MEDIA: files were never extracted or delivered — the user just saw
the raw MEDIA:/path/to/file text.
Fix: after streaming completes, extract MEDIA: tags and local file
paths from the response and deliver them via the platform adapter.
The text is already sent (with the raw tag visible in the stream),
but the actual files now get delivered as attachments.
f58902818d