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_model_config.py

@@ -0,0 +1,30 @@
+from deerflow.config.model_config import ModelConfig
+
+
+def _make_model(**overrides) -> ModelConfig:
+    return ModelConfig(
+        name="openai-responses",
+        display_name="OpenAI Responses",
+        description=None,
+        use="langchain_openai:ChatOpenAI",
+        model="gpt-5",
+        **overrides,
+    )
+
+
+def test_responses_api_fields_are_declared_in_model_schema():
+    assert "use_responses_api" in ModelConfig.model_fields
+    assert "output_version" in ModelConfig.model_fields
+
+
+def test_responses_api_fields_round_trip_in_model_dump():
+    config = _make_model(
+        api_key="$OPENAI_API_KEY",
+        use_responses_api=True,
+        output_version="responses/v1",
+    )
+
+    dumped = config.model_dump(exclude_none=True)
+
+    assert dumped["use_responses_api"] is True
+    assert dumped["output_version"] == "responses/v1"