fix(server-party): fix context leak in GetPendingMessages acknowledgment

Fixed the acknowledgment goroutine in GetPendingMessages to use parent
context instead of context.Background(), preventing orphan goroutines
that can't be cancelled.

This completes all context bug fixes:
- server-party-api event handler (commit 450163a)
- server-party event handler (commit 99ff3ac)
- message acknowledgment in SubscribeMessages (commit 450163a)
- message acknowledgment in GetPendingMessages (this commit)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/mpc-system/services/server-party/adapters/output/grpc/message_router_client.go

@@ -268,7 +268,8 @@ func (c *MessageRouterClient) GetPendingMessages(
 
 			// Send acknowledgment for each received message
 			go func(messageID, sessionIDStr, pID string) {
-				ackCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+				// Use parent context to allow proper cancellation
+				ackCtx, cancel := context.WithTimeout(ctx, 5*time.Second)
 				defer cancel()
 				sid, _ := uuid.Parse(sessionIDStr)
 				if err := c.AcknowledgeMessage(ackCtx, messageID, sid, pID, true, ""); err != nil {