fix(server-party): include self in participants list for keygen

The JoinSession response contains OtherParties (excluding self) and
PartyIndex (self's index). The participants list passed to TSS keygen
must include all parties including self, otherwise validation fails
with "invalid party count" error.

🤖 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

@@ -637,16 +637,28 @@ func (c *MessageRouterClient) JoinSession(
 		}
 
 		// Convert response to SessionInfo
-		participants := make([]use_cases.ParticipantInfo, len(resp.OtherParties))
-		for i, p := range resp.OtherParties {
+		// Include both other parties AND self in the participants list
+		participants := make([]use_cases.ParticipantInfo, 0, len(resp.OtherParties)+1)
+
+		// Add self first
+		participants = append(participants, use_cases.ParticipantInfo{
+			PartyID:    partyID,
+			PartyIndex: int(resp.PartyIndex),
+		})
+		logger.Debug("Added self to participants",
+			zap.String("party_id", partyID),
+			zap.Int32("party_index", resp.PartyIndex))
+
+		// Add other parties
+		for _, p := range resp.OtherParties {
 			logger.Debug("Received party_index from Message Router",
 				zap.String("party_id", p.PartyId),
 				zap.Int32("party_index", p.PartyIndex))
 
-			participants[i] = use_cases.ParticipantInfo{
+			participants = append(participants, use_cases.ParticipantInfo{
 				PartyID:    p.PartyId,
 				PartyIndex: int(p.PartyIndex),
-			}
+			})
 		}
 
 		sessionInfo := &use_cases.SessionInfo{