refactor(scenario): remove validateCode — assertions live in execCode

- drop validateCode column, DTOs, service mappings, and scheduler branch
- remove parseValidateResult helper and ValidateResult interface
- inject playwright expect into code executor for direct use in execCode
- strip validateCode from MCP tool schemas, client types, and UI forms

server/src/scenario/dto/create-scenario-step.dto.ts

@@ -13,12 +13,4 @@ export class CreateScenarioStepDto {
   @IsNotEmpty()
   execCode?: string;
 
-  @ApiPropertyOptional({
-    example:
-      'return { success: result !== null, description: "title present" };',
-  })
-  @IsOptional()
-  @IsString()
-  @IsNotEmpty()
-  validateCode?: string;
 }