feat(community): add SearXNG and Browserless web search/fetch tools (#3451)

* feat(community): add SearXNG and Browserless web search/fetch tools

- SearXNG web_search: privacy-focused meta search engine integration
  with configurable base_url via config.yaml tool settings
- Browserless web_fetch: headless browser page fetching with
  readability article extraction
- Both tools are fully configurable through tool config section
- No external API keys required for basic operation

* fix: address PR review feedback and add unit tests

- Guard config.model_extra against None values (review #1, #2)
- Coerce max_results to int when reading from config (review #2)
- Fix web_fetch_tool to use direct HTTP fetch instead of reusing
  the web_search client config (review #3)
- Fix misleading docstring for SearxngClient.fetch (review #4)
- Remove unused target_url variable to pass Ruff lint (review #5)
- Normalize bool config values with _normalize_bool helper to
  handle env-resolved string values correctly (review #6)
- Add unit tests for both SearXNG and Browserless client classes
  and their tool functions with mocked httpx (review #7, #8)

* fix: convert to async httpx to avoid blocking I/O on event loop

- Replace httpx.Client with httpx.AsyncClient in both client classes
- Convert tool functions to async def
- Wrap readability_extractor calls in asyncio.to_thread()
- Update all tests to use pytest.mark.asyncio and async mocks
- Fix import sorting to pass Ruff lint

* fix(browserless): replace deprecated waitUntil with waitForEvent

The Browserless API has deprecated the waitUntil parameter.
Replace with waitForEvent which accepts values like 'networkidle'.
Default is empty (no wait), configurable via config.yaml.

* fix(browserless): remove deprecated gotoTimeout and bestAttempt params

The Browserless /content API does not accept gotoTimeout or bestAttempt
as top-level payload keys. These were being sent unconditionally,
causing 400 Bad Request errors on current Browserless versions.

Changes:
- Remove goto_timeout_ms parameter and 'gotoTimeout' from payload
- Remove best_attempt parameter and 'bestAttempt' from payload
- Remove _normalize_bool helper (no longer needed)
- Remove goto_timeout_ms and best_attempt config reading in tools.py
- Add tests for waitForSelector and reject params
- Verify no deprecated params are sent in test_fetch_html_success

* refactor(searxng): remove web_fetch_tool, decouple from web_search config

SearXNG is a search engine — it should only provide web_search_tool.
The web_fetch responsibility belongs to Browserless (headless Chrome)
or Jina AI, not SearXNG.

Changes:
- Remove web_fetch_tool from SearXNG tools.py and __init__.py
- Remove SearxngClient.fetch() method (no longer needed)
- Remove unused asyncio/readability imports from SearXNG tools.py
- Add test for max_results string-to-int coercion from config
- Add test for search with categories parameter
- Add test for httpx.RequestError handling
- Apply ruff format fixes to browserless_client.py and test files

backend/packages/harness/deerflow/community/browserless/__init__.py

@@ -0,0 +1,4 @@
+from .browserless_client import BrowserlessClient
+from .tools import web_fetch_tool
+
+__all__ = ["BrowserlessClient", "web_fetch_tool"]