fix(co-sign): allow signing sessions with t participants instead of n

- Modify ValidateSessionCreation to differentiate between keygen and sign sessions
- For keygen: require participantCount == threshold.N() (all parties must participate)
- For sign: require participantCount == threshold.T() (only t parties needed)
- This fixes "session is full" error when creating signing session with 3 parties but n=5

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

backend/mpc-system/services/session-coordinator/domain/services/session_coordinator.go

@@ -27,9 +27,23 @@ func (s *SessionCoordinatorService) ValidateSessionCreation(
 		return entities.ErrInvalidSessionType
 	}
 
-	// Allow either exact participant count (pre-registered) or 0 (dynamic joining)
-	if participantCount != 0 && participantCount != threshold.N() {
-		return entities.ErrSessionFull
+	// Validate participant count based on session type
+	// For keygen: all n parties must participate (participantCount == n or 0 for dynamic)
+	// For sign: only t parties participate (participantCount == t or 0 for dynamic)
+	if participantCount != 0 {
+		if sessionType == entities.SessionTypeSign {
+			// Signing session: participant count should equal threshold t
+			// (the minimum number of parties needed to sign)
+			if participantCount != threshold.T() {
+				return entities.ErrSessionFull
+			}
+		} else {
+			// Keygen session: participant count should equal threshold n
+			// (all parties must participate in key generation)
+			if participantCount != threshold.N() {
+				return entities.ErrSessionFull
+			}
+		}
 	}
 
 	if sessionType == entities.SessionTypeSign && len(messageHash) == 0 {