fix(proto): add party_index to JoinSessionResponse for correct index assignment

The JoinSessionResponse from coordinator was missing party_index field,
causing message router to try finding self's index in OtherParties (which
only contains other parties). This resulted in incorrect party index
assignment leading to "duplicate indexes" error in TSS keygen.

Changes:
- Add party_index field to coordinator's JoinSessionResponse proto
- Coordinator now includes PartyIndex in gRPC response
- Message router uses party_index from coordinator instead of searching

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

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

backend/mpc-system/api/proto/session_coordinator.proto

@@ -82,6 +82,7 @@ message JoinSessionResponse {
   bool success = 1;
   SessionInfo session_info = 2;
   repeated PartyInfo other_parties = 3;
+  int32 party_index = 4;                   // This party's assigned index
 }
 
 // SessionInfo contains session information

backend/mpc-system/services/message-router/adapters/input/grpc/message_grpc_handler.go

@@ -518,6 +518,9 @@ func (s *MessageRouterServer) JoinSession(
 		}
 	}
 
+	// Use party_index directly from coordinator response
+	resp.PartyIndex = coordResp.PartyIndex
+
 	if len(coordResp.OtherParties) > 0 {
 		resp.OtherParties = make([]*pb.PartyInfo, len(coordResp.OtherParties))
 		for i, p := range coordResp.OtherParties {
@@ -525,16 +528,13 @@ func (s *MessageRouterServer) JoinSession(
 				PartyId:    p.PartyId,
 				PartyIndex: p.PartyIndex,
 			}
-			// Find this party's index
-			if p.PartyId == req.PartyId {
-				resp.PartyIndex = p.PartyIndex
-			}
 		}
 	}
 
 	logger.Info("Proxied JoinSession to coordinator",
 		zap.String("session_id", req.SessionId),
 		zap.String("party_id", req.PartyId),
+		zap.Int32("party_index", resp.PartyIndex),
 		zap.Bool("success", resp.Success))
 
 	return resp, nil

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

@@ -251,6 +251,7 @@ func (s *SessionCoordinatorServer) JoinSession(
 			Status:      output.SessionInfo.Status,
 		},
 		OtherParties: otherParties,
+		PartyIndex:   int32(output.PartyIndex),
 	}, nil
 }