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

client/src/api/client.ts

@@ -274,14 +274,12 @@ export const scenarioCredentials = {
 export interface CreateStepPayload {
   title?: string;
   execCode?: string;
-  validateCode?: string;
 }
 
 export interface UpdateStepPayload {
   title?: string;
   order?: number;
   execCode?: string;
-  validateCode?: string;
 }
 
 export const steps = {