fix(server-party): heartbeat during waitForAllParticipants

Problem:
- co_managed_keygen server-party waits for external party after joining
- No heartbeat sent during wait period (up to 5 minutes)
- session-coordinator has 120 second inactivity timeout
- Server-party marked as timed_out/failed while waiting

Fix:
- Send heartbeat in waitForAllParticipants polling loop
- Add Heartbeat method to MessageRouterClient interface
- Heartbeat every 2 seconds with poll interval
- Heartbeat failure only logs warning, does not block

Generated with Claude Code

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/mpc-system/services/server-party/application/use_cases/participate_keygen.go

@@ -56,6 +56,7 @@ type SessionStatusInfo struct {
 type MessageRouterClient interface {
 	RouteMessage(ctx context.Context, sessionID uuid.UUID, fromParty string, toParties []string, roundNumber int, payload []byte) error
 	SubscribeMessages(ctx context.Context, sessionID uuid.UUID, partyID string) (<-chan *MPCMessage, error)
+	Heartbeat(ctx context.Context, partyID string) (int32, error)
 }
 
 // SessionInfo contains session information from coordinator
@@ -127,7 +128,7 @@ func (uc *ParticipateKeygenUseCase) Execute(
 	// For co_managed_keygen: wait for all N participants to join before proceeding
 	// This is necessary because server parties join immediately but external party joins later
 	if sessionInfo.SessionType == "co_managed_keygen" {
-		sessionInfo, err = uc.waitForAllParticipants(ctx, input.SessionID, sessionInfo)
+		sessionInfo, err = uc.waitForAllParticipants(ctx, input.SessionID, sessionInfo, input.PartyID)
 		if err != nil {
 			return nil, err
 		}
@@ -395,6 +396,7 @@ func (uc *ParticipateKeygenUseCase) waitForAllParticipants(
 	ctx context.Context,
 	sessionID uuid.UUID,
 	initialSessionInfo *SessionInfo,
+	partyID string,
 ) (*SessionInfo, error) {
 	logger.Info("Waiting for all participants to join co_managed_keygen session",
 		zap.String("session_id", sessionID.String()),
@@ -418,6 +420,17 @@ func (uc *ParticipateKeygenUseCase) waitForAllParticipants(
 		case <-ctx.Done():
 			return nil, ctx.Err()
 		case <-time.After(pollInterval):
+			// Send heartbeat to keep the party alive during wait
+			// This prevents the session-coordinator from timing out this party
+			_, heartbeatErr := uc.messageRouter.Heartbeat(ctx, partyID)
+			if heartbeatErr != nil {
+				logger.Warn("Failed to send heartbeat during wait",
+					zap.String("session_id", sessionID.String()),
+					zap.String("party_id", partyID),
+					zap.Error(heartbeatErr))
+				// Continue anyway - heartbeat failure is not fatal
+			}
+
 			// Get full session status including participants
 			statusInfo, err := uc.sessionClient.GetSessionStatusFull(ctx, sessionID)
 			if err != nil {