Initial commit: hardened DeerFlow factory

Vendored deer-flow upstream (bytedance/deer-flow) plus prompt-injection
hardening:

- New deerflow.security package: content_delimiter, html_cleaner,
  sanitizer (8 layers — invisible chars, control chars, symbols, NFC,
  PUA, tag chars, horizontal whitespace collapse with newline/tab
  preservation, length cap)
- New deerflow.community.searx package: web_search, web_fetch,
  image_search backed by a private SearX instance, every external
  string sanitized and wrapped in <<<EXTERNAL_UNTRUSTED_CONTENT>>>
  delimiters
- All native community web providers (ddg_search, tavily, exa,
  firecrawl, jina_ai, infoquest, image_search) replaced with hard-fail
  stubs that raise NativeWebToolDisabledError at import time, so a
  misconfigured tool.use path fails loud rather than silently falling
  back to unsanitized output
- Native client back-doors (jina_client.py, infoquest_client.py)
  stubbed too
- Native-tool tests quarantined under tests/_disabled_native/
  (collect_ignore_glob via local conftest.py)
- Sanitizer Layer 7 fix: only collapse horizontal whitespace, preserve
  newlines and tabs so list/table structure survives
- Hardened runtime config.yaml references only the searx-backed tools
- Factory overlay (backend/) kept in sync with deer-flow tree as a
  reference / source

See HARDENING.md for the full audit trail and verification steps.

deer-flow/backend/tests/test_checkpointer_none_fix.py

@@ -0,0 +1,54 @@
+"""Test for issue #1016: checkpointer should not return None."""
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+from langgraph.checkpoint.memory import InMemorySaver
+
+
+class TestCheckpointerNoneFix:
+    """Tests that checkpointer context managers return InMemorySaver instead of None."""
+
+    @pytest.mark.anyio
+    async def test_async_make_checkpointer_returns_in_memory_saver_when_not_configured(self):
+        """make_checkpointer should return InMemorySaver when config.checkpointer is None."""
+        from deerflow.agents.checkpointer.async_provider import make_checkpointer
+
+        # Mock get_app_config to return a config with checkpointer=None
+        mock_config = MagicMock()
+        mock_config.checkpointer = None
+
+        with patch("deerflow.agents.checkpointer.async_provider.get_app_config", return_value=mock_config):
+            async with make_checkpointer() as checkpointer:
+                # Should return InMemorySaver, not None
+                assert checkpointer is not None
+                assert isinstance(checkpointer, InMemorySaver)
+
+                # Should be able to call alist() without AttributeError
+                # This is what LangGraph does and what was failing in issue #1016
+                result = []
+                async for item in checkpointer.alist(config={"configurable": {"thread_id": "test"}}):
+                    result.append(item)
+
+                # Empty list is expected for a fresh checkpointer
+                assert result == []
+
+    def test_sync_checkpointer_context_returns_in_memory_saver_when_not_configured(self):
+        """checkpointer_context should return InMemorySaver when config.checkpointer is None."""
+        from deerflow.agents.checkpointer.provider import checkpointer_context
+
+        # Mock get_app_config to return a config with checkpointer=None
+        mock_config = MagicMock()
+        mock_config.checkpointer = None
+
+        with patch("deerflow.agents.checkpointer.provider.get_app_config", return_value=mock_config):
+            with checkpointer_context() as checkpointer:
+                # Should return InMemorySaver, not None
+                assert checkpointer is not None
+                assert isinstance(checkpointer, InMemorySaver)
+
+                # Should be able to call list() without AttributeError
+                result = list(checkpointer.list(config={"configurable": {"thread_id": "test"}}))
+
+                # Empty list is expected for a fresh checkpointer
+                assert result == []