feat(scenario): export/import endpoints and integration tests

- add GET /scenarios/:id/export returning ScenarioExportDto
- add POST /scenarios/import creating scenario with all steps
- add ScenarioExportDto and ScenarioStepExportDto classes
- fix McpServer singleton by creating fresh instance per handle() call
- add 12 integration tests covering shape, ordering, round-trip, validation

src/scenario/dto/scenario-export.dto.ts

@@ -0,0 +1,54 @@
+import { ApiProperty, ApiPropertyOptional } from '@nestjs/swagger';
+import { Type } from 'class-transformer';
+import {
+  IsArray,
+  IsIn,
+  IsInt,
+  IsNotEmpty,
+  IsOptional,
+  IsString,
+  Min,
+  ValidateNested,
+} from 'class-validator';
+import { StepType } from '../scenario-step.entity';
+
+export class ScenarioStepExportDto {
+  @ApiProperty()
+  @IsInt()
+  @Min(0)
+  order: number;
+
+  @ApiProperty({ enum: ['login', 'exec', 'sign'] })
+  @IsIn(['login', 'exec', 'sign'])
+  type: StepType;
+
+  @ApiProperty()
+  @IsString()
+  @IsNotEmpty()
+  sessionName: string;
+
+  @ApiPropertyOptional()
+  @IsOptional()
+  @IsString()
+  @IsNotEmpty()
+  execCode: string | null;
+
+  @ApiPropertyOptional()
+  @IsOptional()
+  @IsString()
+  @IsNotEmpty()
+  validateCode: string | null;
+}
+
+export class ScenarioExportDto {
+  @ApiProperty()
+  @IsString()
+  @IsNotEmpty()
+  name: string;
+
+  @ApiProperty({ type: [ScenarioStepExportDto] })
+  @IsArray()
+  @ValidateNested({ each: true })
+  @Type(() => ScenarioStepExportDto)
+  steps: ScenarioStepExportDto[];
+}