feat: add pastParticipants support in history sync

Surfaces past group participant data from WhatsApp history sync messages.
Previously pastParticipants was dropped during event buffering because
it was overwritten instead of accumulated across chunks.

- src/Types/Events.ts: add pastParticipants to BaileysEventMap['messaging-history.set'] and BufferedEventData.historySets
- src/Utils/event-buffer.ts: accumulate pastParticipants via spread array instead of overwriting; include in consolidateEvents output
- src/Utils/history.ts: pass item.pastParticipants in processHistoryMessage return

Refs: WhiskeySockets/Baileys#2426

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Utils/event-buffer.ts

@@ -967,6 +967,10 @@ function append<E extends BufferableEvent>(
 				}
 			}
 
+			data.historySets.pastParticipants = [
+				...(data.historySets.pastParticipants || []),
+				...(eventData.pastParticipants || [])
+			]
 			data.historySets.empty = false
 			data.historySets.syncType = eventData.syncType
 			data.historySets.progress = eventData.progress
@@ -1292,6 +1296,7 @@ function consolidateEvents(data: BufferedEventData) {
 			chats: Object.values(data.historySets.chats),
 			messages: Object.values(data.historySets.messages),
 			contacts: Object.values(data.historySets.contacts),
+			pastParticipants: data.historySets.pastParticipants,
 			syncType: data.historySets.syncType,
 			progress: data.historySets.progress,
 			isLatest: data.historySets.isLatest,

src/Utils/history.ts

@@ -378,6 +378,7 @@ export const processHistoryMessage = (item: proto.IHistorySync, logger?: ILogger
 		chats,
 		contacts,
 		messages,
+		pastParticipants: item.pastParticipants,
 		lidPnMappings,
 		syncType: item.syncType,
 		progress: item.progress