Three fixes for the Matrix adapter:
1. Remove RoomMessageMedia callback registration — RoomMessageImage
inherits from it, causing images to be processed twice.
2. Add event ID deduplication to both text and media handlers.
nio can fire the same event more than once; bounded deque+set
tracks the last 1000 events.
3. Cache images locally via Matrix client download. MXC URLs require
authentication, so the vision pipeline couldn't access them.
Images are now downloaded via the authenticated client and saved
to the local cache (same pattern as Telegram/Discord).
Cherry-picked from PR #2353 by williamtwomey.
Co-authored-by: williamtwomey <williamtwomey@users.noreply.github.com>
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