fix(mpc-service): 确保 keygen 会话包含完整的参与者列表

问题：account-service 要求 participants 数量必须等于 threshold_n
原因：createKeygenSession 传入的 participants 可能不足 3 个

修复：
- 在 createKeygenSession 中自动补全参与者列表
- 对于 2-of-3 配置，确保有 3 个参与者：
  - user-party (用户端)
  - server-party-1 (服务端)
  - server-party-2 (备份)

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

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

backend/services/mpc-service/src/infrastructure/external/tss-lib/tss-wrapper.ts

@@ -255,20 +255,51 @@ export class TSSWrapper implements TSSProtocolDomainService {
   /**
    * Create a keygen session via account-service.
    * This will also notify server-party-1/2/3 to participate.
+   *
+   * Note: account-service requires exactly threshold_n participants.
+   * For a 2-of-3 setup, we need 3 participants:
+   * - user-party (the user's share, returned to client)
+   * - server-party-1 (stored on server)
+   * - server-party-2 (backup, stored on server)
    */
   private async createKeygenSession(
     participants: TSSParticipant[],
     threshold: Threshold,
   ): Promise<CreateKeygenSessionResponse> {
+    // Build the full participant list for threshold_n parties
+    // If we have fewer participants, add the default server parties
+    const allParticipants: Array<{ party_id: string; device_type: string }> = [];
+
+    // Add provided participants
+    for (const p of participants) {
+      allParticipants.push({
+        party_id: p.partyId,
+        device_type: 'server',
+      });
+    }
+
+    // Ensure we have exactly threshold_n participants
+    // Default server party IDs for 2-of-3 setup
+    const defaultPartyIds = ['user-party', 'server-party-1', 'server-party-2'];
+    const existingPartyIds = new Set(allParticipants.map(p => p.party_id));
+
+    for (const partyId of defaultPartyIds) {
+      if (!existingPartyIds.has(partyId) && allParticipants.length < threshold.n) {
+        allParticipants.push({
+          party_id: partyId,
+          device_type: partyId === 'user-party' ? 'client' : 'server',
+        });
+      }
+    }
+
+    this.logger.log(`Creating keygen session with ${allParticipants.length} participants: ${allParticipants.map(p => p.party_id).join(', ')}`);
+
     const response = await this.axiosClient.post<CreateKeygenSessionResponse>(
       `${this.accountServiceUrl}/api/v1/mpc/keygen`,
       {
         threshold_n: threshold.n,
         threshold_t: threshold.t,
-        participants: participants.map(p => ({
-          party_id: p.partyId,
-          device_type: 'server',
-        })),
+        participants: allParticipants,
       },
     );
     return response.data;