Assistant: stop stale clients from resurrecting cleared threads

Root cause of the "chunks" clear bug: conversation history is
server-owned, but MAF seeds a thread from whatever the client sends
when no snapshot exists. Any stale tab (open across a clear or from an
older session) still holds the old conversation in memory, and its next
question re-persisted the whole thing — clears appeared to peel off one
layer at a time.

The agent middleware now drops client-sent history for threads with no
stored snapshot, keeping only the last user turn (fresh conversations
and empty-messages hydration requests pass through untouched). Verified
live: simulated a cleared-thread stale tab, asked from it, and only the
new turn was persisted.

Also: clearThread surfaces DELETE failures as a toast instead of
silently reloading, and conftest sets a dummy OPENAI_API_KEY so tests
can import app.main.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/pages/Asistente.tsx

@@ -8,6 +8,7 @@ import {
 } from "@copilotkit/react-core/v2";
 import { useCopilotAction } from "@copilotkit/react-core";
 import { MessageSquarePlus, Sparkles } from "lucide-react";
+import { toast } from "sonner";
 import { Button } from "@/components/ui/button";
 import ConfirmDialog from "@/components/ConfirmDialog";
 import { PageHeader } from '@/components/page-header';
@@ -78,10 +79,13 @@ export default function Asistente() {
   const clearThread = async () => {
     setClearing(true);
     try {
-      await fetch("/api/v1/chat/thread", {
+      const res = await fetch("/api/v1/chat/thread", {
         method: "DELETE",
         credentials: "include",
       });
+      if (!res.ok) {
+        throw new Error(`DELETE /chat/thread → ${res.status}`);
+      }
       // Full reload is the reliable reset: CopilotKit keeps chat state in
       // more places than agent.messages (clearing it in place left the
       // conversation rendered), while a fresh boot hydrates from the
@@ -89,7 +93,9 @@ export default function Asistente() {
       window.location.reload();
     } catch (err) {
       console.error("[asistente] clear thread failed:", err);
+      toast.error("No se pudo borrar la conversación. Intenta de nuevo.");
       setClearing(false);
+      setConfirmClear(false);
     }
   };