feat: add KeygenSessionID to MPCSession entity

- Add KeygenSessionID field to MPCSession struct for tracking which keygen's shares to use
- This is the first step in完整的修复流程

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

@@ -42,9 +42,10 @@ type MPCSession struct {
 	Threshold       value_objects.Threshold
 	Participants    []*Participant
 	Status          value_objects.SessionStatus
-	MessageHash     []byte // Used for Sign sessions
-	PublicKey       []byte // Group public key after Keygen completion
-	DelegatePartyID string // The delegate party ID (returns share to user instead of storing)
+	MessageHash     []byte    // Used for Sign sessions
+	PublicKey       []byte    // Group public key after Keygen completion
+	DelegatePartyID string    // The delegate party ID (returns share to user instead of storing)
+	KeygenSessionID uuid.UUID // For Sign sessions: which keygen session's shares to use
 	CreatedBy       string
 	CreatedAt       time.Time
 	UpdatedAt       time.Time