Fix: Improvements for security around the AI (#12895)

* Fix: Validate and limit chat question input in ChatStreamingView

Add max_length=4000 to ChatStreamingSerializer.q and replace the bare
request.data["q"] read with proper serializer.is_valid(raise_exception=True)
so oversized or missing questions are rejected with HTTP 400 before
reaching the LLM.

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

* Fix: Add defensive prompt framing to mark document content as untrusted

* Also adds a system prompt which is treated higher that this is untrusted stuff

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

src/documents/tests/test_api_chat.py

@@ -0,0 +1,44 @@
+from __future__ import annotations
+
+from unittest import mock
+
+from django.contrib.auth.models import User
+from rest_framework import status
+from rest_framework.test import APITestCase
+
+
+class TestChatStreamingViewInputValidation(APITestCase):
+    def setUp(self) -> None:
+        super().setUp()
+        self.user = User.objects.create_superuser(username="temp_admin")
+        self.client.force_authenticate(user=self.user)
+
+    def _mock_ai_enabled(self) -> mock.MagicMock:
+        """Return a mock AIConfig instance with ai_enabled=True."""
+        m = mock.MagicMock()
+        m.ai_enabled = True
+        return m
+
+    def test_oversized_question_is_rejected(self) -> None:
+        with mock.patch(
+            "documents.views.AIConfig",
+            return_value=self._mock_ai_enabled(),
+        ):
+            resp = self.client.post(
+                "/api/documents/chat/",
+                {"q": "x" * 4001},
+                format="json",
+            )
+        assert resp.status_code == status.HTTP_400_BAD_REQUEST
+
+    def test_missing_question_is_rejected(self) -> None:
+        with mock.patch(
+            "documents.views.AIConfig",
+            return_value=self._mock_ai_enabled(),
+        ):
+            resp = self.client.post(
+                "/api/documents/chat/",
+                {},
+                format="json",
+            )
+        assert resp.status_code == status.HTTP_400_BAD_REQUEST