fix(identity-service): 修复 MPC keygen 请求参数

- sessionId 使用纯 UUID 格式（移除 mpc- 前缀）
- shareType 使用 'wallet' 匹配 PartyShareType 枚举

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

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

backend/services/identity-service/src/infrastructure/external/mpc/mpc-client.service.ts

@@ -79,10 +79,10 @@ export class MpcClientService {
   }
 
   /**
-   * 生成新的会话ID
+   * 生成新的会话ID (必须是纯 UUID 格式)
    */
   generateSessionId(): string {
-    return `mpc-${randomUUID()}`;
+    return randomUUID();
   }
 
   /**
@@ -112,7 +112,7 @@ export class MpcClientService {
             sessionId: request.sessionId,
             partyId: 'server-party',
             joinToken: this.generateJoinToken(request.sessionId),
-            shareType: 'SERVER',
+            shareType: 'wallet',  // PartyShareType.WALLET
             userId: request.parties.find(p => p.partyType === 'CLIENT')?.partyId,
           },
           {