fix(mpc-service): 添加 DTO 验证装饰器

添加 class-validator 装饰器到 CreateKeygenDto 和 CreateSigningDto，
修复 NestJS ValidationPipe 的 forbidNonWhitelisted 验证错误。

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

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

backend/services/mpc-service/src/api/controllers/mpc.controller.ts

@@ -28,6 +28,7 @@ import {
   ApiResponse,
   ApiParam,
 } from '@nestjs/swagger';
+import { IsString, IsNumber, IsBoolean, IsOptional } from 'class-validator';
 import { Public } from '../../shared/decorators/public.decorator';
 import { MPCCoordinatorService } from '../../application/services/mpc-coordinator.service';
 
@@ -36,15 +37,28 @@ import { MPCCoordinatorService } from '../../application/services/mpc-coordinato
 // ============================================
 
 export class CreateKeygenDto {
+  @IsString()
   username: string;
+
+  @IsNumber()
   thresholdN: number;
+
+  @IsNumber()
   thresholdT: number;
+
+  @IsBoolean()
   requireDelegate: boolean;
 }
 
 export class CreateSigningDto {
+  @IsString()
   username: string;
+
+  @IsString()
   messageHash: string;
+
+  @IsString()
+  @IsOptional()
   userShare?: string;
 }