Fix teleguard Messages media: skip cache dirs, drop unresolved refs (LAVA crash)

[abrignoni]

Bug reported while testing the converted Teleguard - Messages artifact in LAVA: the Media column showed a red broken-media marker and the viewer crashed (black screen) on hover, even though the images rendered fine in the HTML report.

Root cause
The media matcher used a loose substring match (key in str(f)). TeleGuard stores each attachment at cache/<key>/<file>, so the metadata key is a sub-path, and the substring also matched the cache/<key> directory. check_in_media() returns None for a directory, and that None was appended into the media list. lava_insert_sqlite_data json.dumps's a list cell, so it became ["validref", null]; the LAVA viewer chokes on the null (broken marker + crash on hover). The HTML path (get_data_list_with_media) silently skips unresolved refs, which is why it only reproduced in LAVA.

Fix

• Require os.path.isfile on the match, so the cache/<key> directory is never selected.
• Only append the ref if check_in_media(...) returned one (no None in the cell).
• Collapse a single-attachment message to a bare ref (renders like any other single-media cell); keep a list only for genuine multi-attachment messages.

Kept the substring match itself (not exact-basename) because the key is a sub-path, not a filename — exact-basename matches nothing and the media goes N/A.

Verification

• Fixture test on a real teleguard_database.db modelling the cache/<key>/<file> layout (inner basename != key) with the cache directories present in files_found: resolves the inner file, skips both directories, never emits None, single attachment → bare ref, missing/text rows → empty.
• pylint clean (-d C,R); PluginLoader loads 492.
• Reporter confirmed images now render in both the HTML report and the LAVA Media column, no crash.