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/pages/scenario/EditStepPage.tsx

@@ -17,7 +17,6 @@ export function EditStepPage() {
   const [step, setStep] = useState<ScenarioStep | null>(null);
   const [title, setTitle] = useState('');
   const [execCode, setExecCode] = useState('');
-  const [validateCode, setValidateCode] = useState('');
   const [loading, setLoading] = useState(true);
   const [saving, setSaving] = useState(false);
   const [error, setError] = useState<string | null>(null);
@@ -30,7 +29,6 @@ export function EditStepPage() {
         setStep(st);
         setTitle(st.title ?? '');
         setExecCode(st.execCode ?? '');
-        setValidateCode(st.validateCode ?? '');
       })
       .catch((err: Error) => setError(err.message))
       .finally(() => setLoading(false));
@@ -44,7 +42,6 @@ export function EditStepPage() {
       await steps.update(id!, stepId!, {
         title: title.trim() || undefined,
         execCode: execCode.trim() || undefined,
-        validateCode: validateCode.trim() || undefined,
       });
       toast.success('Step updated');
       navigate(`/scenarios/${id}`);
@@ -88,10 +85,6 @@ export function EditStepPage() {
                 <label className={styles.fieldLabel}>{t('steps.form_exec_code')}</label>
                 <CodeEditor value={execCode} onChange={setExecCode} rows={6} />
               </div>
-              <div className={styles.formField}>
-                <label className={styles.fieldLabel}>{t('steps.form_validate_code')}</label>
-                <CodeEditor value={validateCode} onChange={setValidateCode} rows={6} />
-              </div>
             </div>
 
             <div className={styles.formActions}>