fix(co-sign): fix CanStart() to check T parties for sign sessions

- For keygen sessions: require all N parties to join before starting
- For sign sessions: require only T parties to join before starting
- This fixes session_started event not being triggered for signing sessions

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

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

backend/mpc-system/services/session-coordinator/domain/entities/mpc_session.go

@@ -140,7 +140,15 @@ func (s *MPCSession) UpdateParticipantStatus(partyID value_objects.PartyID, stat
 
 // CanStart checks if all participants have joined and the session can start
 func (s *MPCSession) CanStart() bool {
-	if len(s.Participants) != s.Threshold.N() {
+	// Determine required participant count based on session type
+	// For keygen: all N parties must participate
+	// For sign: only T parties participate
+	requiredCount := s.Threshold.N()
+	if s.SessionType == SessionTypeSign {
+		requiredCount = s.Threshold.T()
+	}
+
+	if len(s.Participants) != requiredCount {
 		return false
 	}
 
@@ -151,7 +159,7 @@ func (s *MPCSession) CanStart() bool {
 			readyCount++
 		}
 	}
-	return readyCount == s.Threshold.N()
+	return readyCount == requiredCount
 }
 
 // Start transitions the session to in_progress