fix(messages-recv): preserve custom WAMessageKey fields after proto roundtrip

proto.WebMessageInfo.fromObject() rebuilds the key from proto.MessageKey
schema, which only declares remoteJid/fromMe/id/participant. Custom
Baileys extensions (participantAlt, participantUsername, addressingMode)
are silently dropped, so process-message.ts cannot read them downstream
and authorUsername/authorPn are lost on group-participants.update,
groups.update, and group.join-request events that originate from
notifications.

Re-attach msg.key after the roundtrip to preserve the custom fields.
Addresses Codex P2 review comment on PR #382.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Socket/messages-recv.ts

@@ -2232,6 +2232,10 @@ export const makeMessagesRecvSocket = (config: SocketConfig) => {
 						msg.messageTimestamp = +node.attrs.t!
 
 						const fullMsg = proto.WebMessageInfo.fromObject(msg) as WAMessage
+						// Preserve custom WAMessageKey fields (participantAlt, participantUsername,
+						// addressingMode) that proto.WebMessageInfo.fromObject strips because
+						// they aren't part of the proto.MessageKey schema.
+						fullMsg.key = msg.key
 						await upsertMessage(fullMsg, 'append')
 					}
 				})