feat(mpc-system): optimize party index handling and add gRPC debug logs

- Simplified participant list handling in JoinSession client
- Added debug logging for party_index conversion in gRPC messages
- Removed redundant party filtering logic
- Added detailed logging to trace protobuf field values

🤖 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/session_coordinator_client.go

@@ -80,21 +80,18 @@ func (c *SessionCoordinatorClient) JoinSession(
 	}
 
 	// Convert response to SessionInfo
-	participants := make([]use_cases.ParticipantInfo, 0, len(resp.OtherParties)+1)
+	// Note: OtherParties should include ALL participants (including self) from coordinator
+	participants := make([]use_cases.ParticipantInfo, len(resp.OtherParties))
+	for i, p := range resp.OtherParties {
+		// Debug: Log what we received from gRPC
+		logger.Info("gRPC client - received party_index from protobuf response",
+			zap.String("party_id", p.PartyId),
+			zap.Int32("proto_party_index", p.PartyIndex),
+			zap.Int("converted_party_index", int(p.PartyIndex)))
 
-	// Add self
-	participants = append(participants, use_cases.ParticipantInfo{
-		PartyID:    partyID,
-		PartyIndex: findPartyIndex(resp.OtherParties, partyID),
-	})
-
-	// Add other parties
-	for _, p := range resp.OtherParties {
-		if p.PartyId != partyID {
-			participants = append(participants, use_cases.ParticipantInfo{
-				PartyID:    p.PartyId,
-				PartyIndex: int(p.PartyIndex),
-			})
+		participants[i] = use_cases.ParticipantInfo{
+			PartyID:    p.PartyId,
+			PartyIndex: int(p.PartyIndex),
 		}
 	}
 

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

@@ -150,6 +150,11 @@ func (s *SessionCoordinatorServer) JoinSession(
 				AppVersion: p.DeviceInfo.AppVersion,
 			},
 		}
+		// Debug: Log what we're about to send in gRPC response
+		logger.Info("gRPC JoinSession - setting party_index in protobuf response",
+			zap.String("party_id", p.PartyID),
+			zap.Int("source_party_index", p.PartyIndex),
+			zap.Int32("proto_party_index", otherParties[i].PartyIndex))
 	}
 
 	return &pb.JoinSessionResponse{