fix: use session_id instead of id in AI flow builder

CreateFlowDropdown accessed `session.id` which is undefined — the API
returns `session_id`. The undefined value caused "undefined" to be
interpolated into URL paths, triggering 422 validation errors from
FastAPI.

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

frontend/src/components/common/CreateFlowDropdown.tsx

@@ -30,7 +30,7 @@ export function CreateFlowDropdown({
     const session = await editorAIApi.startSession(
       aiPromptFlowType === 'maintenance' ? 'procedural' : aiPromptFlowType
     )
-    const sessionId = session.id
+    const sessionId = session.session_id
 
     // Send the user's prompt
     await editorAIApi.sendMessage({