feat(community): add Serper web search provider (#2630)

* feat(community): add Serper web search provider

Add a new community search provider backed by the Serper Google Search
API (https://serper.dev). Serper returns real-time Google results via a
simple JSON API and requires only an API key — no extra Python package.

Changes:
- backend/packages/harness/deerflow/community/serper/__init__.py
- backend/packages/harness/deerflow/community/serper/tools.py
  Implements web_search_tool using httpx (already a project dependency).
  API key is read from config.yaml `api_key` field or SERPER_API_KEY env var.
  Follows the same interface / output shape as the existing ddg_search provider.
  Exposes max_results parameter (default 5) with config override logic.
- backend/tests/test_serper_tools.py
  Unit tests covering API key resolution, config overrides, HTTP errors,
  empty results, and parameter passing.
- config.example.yaml: add commented-out Serper example alongside other providers
- .env.example: add SERPER_API_KEY placeholder

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

* Fix the lint error

* Fix the lint error

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

config.example.yaml

@@ -373,6 +373,16 @@ tools:
     use: deerflow.community.ddg_search.tools:web_search_tool
     max_results: 5
 
+  # Web search tool (uses Serper - Google Search API, requires SERPER_API_KEY)
+  # Serper provides real-time Google Search results. Sign up at https://serper.dev
+  # Note: set SERPER_API_KEY in your environment before starting the app, or
+  # uncomment and fill in api_key below (the $VAR syntax is resolved at startup).
+  # - name: web_search
+  #   group: web
+  #   use: deerflow.community.serper.tools:web_search_tool
+  #   max_results: 5
+  #   # api_key: $SERPER_API_KEY  # Optional if SERPER_API_KEY env var is set
+
   # Web search tool (requires Tavily API key)
   # - name: web_search
   #   group: web