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/scenario.service.ts

@@ -124,7 +124,6 @@ export class ScenarioService {
         scenarioId,
         title: dto.title ?? null,
         execCode: dto.execCode ?? null,
-        validateCode: dto.validateCode ?? null,
       }),
     );
     await this.normalizeStepOrder(scenarioId);
@@ -156,7 +155,6 @@ export class ScenarioService {
       order: step.order,
       title: dto.title ?? step.title,
       execCode: dto.execCode ?? step.execCode,
-      validateCode: dto.validateCode ?? step.validateCode,
     });
     await this.stepRepo.save(step);
 
@@ -404,7 +402,6 @@ export class ScenarioService {
       steps: scenario.steps.map((s) => ({
         title: s.title,
         execCode: s.execCode,
-        validateCode: s.validateCode,
       })),
     };
   }
@@ -436,7 +433,6 @@ export class ScenarioService {
           order: index,
           title: s.title ?? null,
           execCode: s.execCode ?? null,
-          validateCode: s.validateCode ?? null,
         }),
       );
       await this.stepRepo.save(steps);