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>

backend/tests/test_chat_persistence.py

@@ -129,3 +129,39 @@ def test_clear_thread_endpoint(monkeypatch, engine, session):
 
     assert clear_thread(session=session, _user="testadmin") == {"cleared": True}
     assert asyncio.run(store.get(scope="default", thread_id="main")) is None
+
+
+def test_drop_stale_client_history(session, monkeypatch, engine):
+    from app.main import _drop_stale_client_history
+
+    history = [
+        {"role": "user", "content": "vieja pregunta"},
+        {"role": "assistant", "content": "vieja respuesta"},
+        {"role": "tool", "content": "x", "toolCallId": "t1"},
+        {"role": "user", "content": "nueva pregunta"},
+    ]
+
+    # No stored snapshot for the thread → stale client: keep last user turn.
+    assert _drop_stale_client_history(session, "main", list(history)) == [
+        {"role": "user", "content": "nueva pregunta"}
+    ]
+
+    # Fresh conversation (no assistant/tool yet) passes through.
+    fresh = [{"role": "user", "content": "hola"}]
+    assert _drop_stale_client_history(session, "main", list(fresh)) == fresh
+
+    # Empty messages (hydration trigger) pass through.
+    assert _drop_stale_client_history(session, "main", []) == []
+
+    # With a stored snapshot the full history is legitimate.
+    store = _store(monkeypatch, engine)
+    asyncio.run(
+        store.save(
+            scope="default",
+            thread_id="main",
+            snapshot=AGUIThreadSnapshot(
+                messages=[{"id": "u1", "role": "user", "content": "hola"}]
+            ),
+        )
+    )
+    assert _drop_stale_client_history(session, "main", list(history)) == history