Collapse the per-type observed-media dispatch into one platform-agnostic
cache_media_bytes() helper in gateway/platforms/base.py. Any adapter can now
hand it raw attachment bytes + a filename/MIME hint; it classifies against the
shared MIME registries, routes to the right cache_*_from_bytes helper,
sandbox-translates the path, and returns a CachedMedia with a ready
context_note(). Telegram's observed-group path shrinks to: size-gate, download,
call the helper, annotate. Also dedupes the addressed-media type ladder into
_media_message_type().
Net: contributor's Telegram-only +595 LOC becomes a +210/-32 production change,
with the reusable primitive available to Discord/Slack/Signal/etc.
Co-authored-by: Glucksberg <markuscontasul@gmail.com>
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