fix: prevent Monaco onChange echo from dismissing parameter stepper

ScriptBodyEditor's onChange fired when the value prop changed externally
(from handleAcceptCandidate inserting placeholders), creating a feedback
loop that reset acceptingCandidateRef before the second useEffect cycle.
Guard onChange to only propagate when the value actually differs from the
current prop.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/script-editor/ScriptBodyEditor.tsx

@@ -1,4 +1,4 @@
-import { useCallback } from 'react'
+import { useCallback, useRef } from 'react'
 import Editor, { type BeforeMount } from '@monaco-editor/react'
 import { resolutionFlowTheme, THEME_ID } from '@/components/tree-editor/code-mode/resolutionFlowTheme'
 import { Spinner } from '@/components/common/Spinner'
@@ -10,11 +10,22 @@ interface Props {
 }
 
 export function ScriptBodyEditor({ value, onChange, disabled }: Props) {
+  const lastValueRef = useRef(value)
+  lastValueRef.current = value
+
   const handleBeforeMount: BeforeMount = useCallback((monaco) => {
     // Register our dark theme if not already defined
     monaco.editor.defineTheme(THEME_ID, resolutionFlowTheme)
   }, [])
 
+  const handleChange = useCallback((v: string | undefined) => {
+    const next = v ?? ''
+    // Only propagate user-initiated edits, not echoes from external value prop changes
+    if (next !== lastValueRef.current) {
+      onChange(next)
+    }
+  }, [onChange])
+
   return (
     <div className="rounded-xl border border-border overflow-hidden">
       <Editor
@@ -22,7 +33,7 @@ export function ScriptBodyEditor({ value, onChange, disabled }: Props) {
         language="powershell"
         theme={THEME_ID}
         value={value}
-        onChange={v => onChange(v ?? '')}
+        onChange={handleChange}
         beforeMount={handleBeforeMount}
         loading={
           <div className="flex h-[300px] items-center justify-center bg-card">