When a Telegram attachment download/cache fails (typically a transient
httpx.ConnectError to Telegram's CDN), the except handler logged a warning
and fell through to handle_message() with empty media and no text — the user
thought the file was delivered, the agent saw a content-less turn with no
signal an attachment was attempted, and the only record was a buried log line.
Adds _surface_media_cache_failure(): replies to the user in Telegram so they
know to retry, and appends an agent-visible notice to event.text via the
existing _append_observed_note channel so the agent knows an attachment was
attempted and failed. No new event fields (structured-event refactor is out
of scope per #23045). Wired into all five cache-failure sites — photo, voice,
audio, video, document — since they shared the identical silent fall-through.
Bug 1 from #23045 (unsupported types routed as fake user messages) no longer
exists on main: the document handler now accepts any file type, so there is no
rejection branch to fix.
Closes#23045
db16854f34