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>

config.example.yaml

@@ -304,6 +304,23 @@ tools:
   #   # Used to limit the scope of search results, only returns content within the specified time range. Set to -1 to disable time filtering
   #   search_time_range: 10
 
+  # Web search tool (uses Exa, requires EXA_API_KEY)
+  # - name: web_search
+  #   group: web
+  #   use: deerflow.community.exa.tools:web_search_tool
+  #   max_results: 5
+  #   search_type: auto  # Options: auto, neural, keyword
+  #   contents_max_characters: 1000
+  #   # api_key: $EXA_API_KEY
+
+  # Web fetch tool (uses Exa)
+  # NOTE: Only one web_fetch provider can be active at a time.
+  # Comment out the Jina AI web_fetch entry below before enabling this one.
+  # - name: web_fetch
+  #   group: web
+  #   use: deerflow.community.exa.tools:web_fetch_tool
+  #   # api_key: $EXA_API_KEY
+
   # Web fetch tool (uses Jina AI reader)
   - name: web_fetch
     group: web