`_handle_media_message` fetched the mxc:// payload (`_download_and_cache_media`)
before `_build_inbound_event` ran `_resolve_message_context`, so an unmentioned
or non-allowlisted room's m.image/m.file was pulled onto the host and then
dropped. Resolve the context first and hand it to `_build_inbound_event` via a
new `ctx` kwarg so the read receipt / thread mark are still applied once.
Same class as the Teams fix in this PR (review follow-up). One invariant test:
unmentioned group media → 0 downloads, mentioned → 1.
7f13d3f43d