send_message with MEDIA:/path to a WhatsApp target previously dropped the
attachment: the WhatsApp branch never passed media_files, the plugin's
_standalone_send accepted the param but only POSTed text, and WhatsApp was
absent from the media-supported platform list.
- send_message_tool: add a Platform.WHATSAPP media block (mirrors Feishu) that
routes media_files through the whatsapp plugin's standalone_sender_fn, and
add whatsapp to the supported-media list strings.
- whatsapp adapter: _standalone_send now sends text first (skipped when the
chunk is media-only), then uploads each file via the bridge /send-media
endpoint with a mediaType derived from extension/is_voice/force_document, so
images/videos/voice arrive as native bubbles instead of documents.
- _bridge_media_type classifier maps ext -> image|video|audio|document.
Closes#19105 (remaining send_message gap). Other items in the report
(inbound video paths, image_generate auto-deliver, history dedup, native
gateway bubbles) already landed on main.
