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.

backend/tests/test_searx_tools.py

@@ -0,0 +1,15 @@
+"""Integration tests for hardened web tools."""
+
+import pytest
+from deerflow.community.searx.tools import web_search_tool
+
+
+class TestHardenedSearxSearch:
+    """Test hardened SearX search against prompt injection."""
+    
+    def test_search_results_are_delimited(self):
+        """Results must be wrapped in security delimiters."""
+        result = web_search_tool("test query")
+        
+        assert "<<<EXTERNAL_UNTRUSTED_CONTENT>>>" in result
+        assert "<<<END_EXTERNAL_UNTRUSTED_CONTENT>>>" in result