`_on_sidecar_message` ran `_normalize_content` (which base64-decodes and writes
inline attachment/voice bytes into the media cache via `_cache_inbound_attachment`)
before the `chat_type == "group" and self.require_mention` check, so an
unmentioned group attachment was persisted and then dropped. Gate on the
user-typed text extracted without touching attachment bytes (`_mention_gate_text`:
text / richlink / group text items), then normalise and strip the wake word only
for messages that pass.
Same class as the Teams fix in this PR (review follow-up). One invariant test:
unmentioned group attachment -> 0 cache writes, mentioned -> 1 and dispatched.
ee9c0f1a32