feat(community): add Exa search as community tool provider (#1357)

* feat(community): add Exa search as community tool provider

Add Exa (exa.ai) as a new community search provider alongside Tavily,
Firecrawl, InfoQuest, and Jina AI. Exa is an AI-native search engine
with neural, keyword, and auto search types.

New files:
- community/exa/tools.py: web_search_tool and web_fetch_tool
- tests/test_exa_tools.py: 10 unit tests with mocked Exa client

Changes:
- pyproject.toml: add exa-py dependency
- config.example.yaml: add commented-out Exa configuration examples

Usage: set `use: deerflow.community.exa.tools:web_search_tool` in
config.yaml and provide EXA_API_KEY.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(community): address PR review comments for Exa tools

- Make _get_exa_client() accept tool_name param so web_fetch reads its own config
- Remove __init__.py to match namespace package pattern of other providers
- Add duplicate tool name warning in config.example.yaml
- Add regression tests for web_fetch config resolution

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Update revision in uv.lock to 3

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/packages/harness/pyproject.toml

@@ -7,6 +7,7 @@ dependencies = [
     "agent-client-protocol>=0.4.0",
     "agent-sandbox>=0.0.19",
     "dotenv>=0.9.9",
+    "exa-py>=1.0.0",
     "httpx>=0.28.0",
     "kubernetes>=30.0.0",
     "langchain>=1.2.3",