fix(session-coordinator): pass PartyComposition from gRPC request to use case

Fixed critical bug where PartyComposition (persistent/delegate party counts) was being sent
by account-service in gRPC request but was not being extracted and passed to the CreateSession
use case, causing delegate party selection to fail.

Changes:
- Extract PartyComposition from protobuf request and pass to CreateSessionInput
- Add logging for party composition values in gRPC handler
- Return delegate_party_id and selected_parties in CreateSessionResponse
- Load session after creation to get delegate party ID

This fixes the issue where require_delegate=true had no effect and all parties selected
were persistent parties instead of 2 persistent + 1 delegate.

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

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

backend/mpc-system/services/session-coordinator/adapters/input/grpc/session_grpc_handler.go

@@ -160,6 +160,19 @@ func (s *SessionCoordinatorServer) CreateSession(
 		ExpiresIn:    time.Duration(req.ExpiresInSeconds) * time.Second,
 	}
 
+	// Add party composition if provided (for automatic party selection)
+	if req.PartyComposition != nil {
+		inputData.PartyComposition = &input.PartyComposition{
+			PersistentCount: int(req.PartyComposition.PersistentCount),
+			DelegateCount:   int(req.PartyComposition.DelegateCount),
+			TemporaryCount:  int(req.PartyComposition.TemporaryCount),
+		}
+		logger.Info("Party composition specified in gRPC request",
+			zap.Int("persistent_count", inputData.PartyComposition.PersistentCount),
+			zap.Int("delegate_count", inputData.PartyComposition.DelegateCount),
+			zap.Int("temporary_count", inputData.PartyComposition.TemporaryCount))
+	}
+
 	// Add delegate user share if provided (for sign sessions with delegate party)
 	if req.DelegateUserShare != nil {
 		inputData.DelegateUserShare = &input.DelegateUserShare{
@@ -180,11 +193,28 @@ func (s *SessionCoordinatorServer) CreateSession(
 		zap.String("session_id", output.SessionID.String()),
 		zap.Int("num_join_tokens", len(output.JoinTokens)))
 
+	// Load session to get delegate party ID
+	session, err := s.sessionRepo.FindByUUID(ctx, output.SessionID)
+	if err != nil {
+		logger.Error("Failed to load session after creation", zap.Error(err))
+		return nil, toGRPCError(err)
+	}
+
+	// Extract selected party IDs from join tokens
+	selectedParties := make([]string, 0, len(output.JoinTokens))
+	for partyID := range output.JoinTokens {
+		if partyID != "*" { // Exclude universal token
+			selectedParties = append(selectedParties, partyID)
+		}
+	}
+
 	// Convert output to response
 	return &pb.CreateSessionResponse{
-		SessionId:  output.SessionID.String(),
-		JoinTokens: output.JoinTokens,
-		ExpiresAt:  output.ExpiresAt.UnixMilli(),
+		SessionId:       output.SessionID.String(),
+		JoinTokens:      output.JoinTokens,
+		ExpiresAt:       output.ExpiresAt.UnixMilli(),
+		SelectedParties: selectedParties,
+		DelegatePartyId: session.DelegatePartyID,
 	}, nil
 }
 

backend/mpc-system/test_create_session.go

@@ -57,9 +57,10 @@ func main() {
 
 	// Create keygen session via account-service
 	sessionData := map[string]interface{}{
-		"threshold_n": 3,
-		"threshold_t": 2,
-		"username":    "admin",
+		"threshold_n":      3,
+		"threshold_t":      2,
+		"username":         "admin",
+		"require_delegate": true,
 	}
 
 	jsonData, err := json.Marshal(sessionData)