fix(account): use only threshold_t parties for signing instead of all active parties

For 2-of-3 threshold signing, only 2 parties should participate in signing, not all 3. This fixes the 'failed to calculate Bob_mid' error that occurred when all parties tried to sign.

Changes:
- Modified CreateSigningSession to select exactly threshold_t parties when no signing config exists
- For 2-of-3: now selects 2 parties instead of all 3
- Added logging to show party selection details

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

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

backend/mpc-system/services/account/adapters/input/http/account_handler.go

@@ -732,12 +732,26 @@ func (h *AccountHTTPHandler) CreateSigningSession(c *gin.Context) {
 			zap.String("username", req.Username),
 			zap.Strings("configured_parties", partyIDs))
 	} else {
-		// Use all active parties (original behavior)
-		partyIDs = allActivePartyIDs
+		// For threshold signing, select minimum required parties (threshold_t)
+		// For 2-of-3, we need exactly 2 parties to sign (not all 3)
+		requiredParties := accountOutput.Account.ThresholdT
+		if len(allActivePartyIDs) < requiredParties {
+			c.JSON(http.StatusBadRequest, gin.H{
+				"error":            "insufficient active parties for signing",
+				"required":         requiredParties,
+				"available":        len(allActivePartyIDs),
+			})
+			return
+		}
 
-		logger.Info("Using all active parties for signing",
+		// Select first 'threshold_t' parties
+		partyIDs = allActivePartyIDs[:requiredParties]
+
+		logger.Info("Using minimum required parties for threshold signing",
 			zap.String("username", req.Username),
-			zap.Strings("active_parties", partyIDs))
+			zap.Int("threshold_t", requiredParties),
+			zap.Int("total_active", len(allActivePartyIDs)),
+			zap.Strings("selected_parties", partyIDs))
 	}
 
 	// Check if any of the selected parties is a delegate