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-step.entity.ts

@@ -32,9 +32,6 @@ export class ScenarioStepEntity {
   @Column({ type: "text", nullable: true })
   execCode: string | null;
 
-  @Column({ type: "text", nullable: true })
-  validateCode: string | null;
-
   @CreateDateColumn()
   createdAt: Date;