fix: address code review findings for AI chat builder

- C1: Fix race condition in handleReset — await abandonSession before
  starting new session to prevent store state corruption
- I1: Extract error messages from Axios response.data.detail instead of
  generic error.message — users now see helpful backend messages (quota
  limits, message caps, etc.)
- I2: Add isGenerating guard in generateTree store action to prevent
  concurrent generation requests on double-click
- I3: Add isResponding guard in sendMessage to prevent concurrent sends
- M5: Remove redundant type casts on flowType
- M6: Add rate limiter to DELETE /sessions/{id} for consistency

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/AIChatBuilderPage.tsx

@@ -40,7 +40,7 @@ export function AIChatBuilderPage() {
     if (resumeId && !sessionId) {
       resumeSession(resumeId)
     } else if (!sessionId && status === 'idle') {
-      startSession(flowType as 'troubleshooting' | 'procedural')
+      startSession(flowType)
     }
   }, []) // eslint-disable-line react-hooks/exhaustive-deps
 
@@ -81,15 +81,15 @@ export function AIChatBuilderPage() {
     }
   }, [importToEditor, treeMetadata, flowType, navigate])
 
-  const handleReset = useCallback(() => {
-    abandonSession()
+  const handleReset = useCallback(async () => {
+    await abandonSession()
     // Clear session from URL
     setSearchParams((prev) => {
       const next = new URLSearchParams(prev)
       next.delete('session')
       return next
     }, { replace: true })
-    startSession(flowType as 'troubleshooting' | 'procedural')
+    startSession(flowType)
   }, [abandonSession, startSession, flowType, setSearchParams])
 
   // Show error toast