feat: add Exa AI-powered search tool

[tgonzalezc5]

Summary

• Adds ExaSearchTool as a new built-in search provider alongside DuckDuckGo and Google Scholar
• Supports multiple search types (auto, neural, fast, instant), content retrieval modes (highlights, full text, summary), category filtering (company, research paper, news, etc.), domain filtering, text filtering, and date ranges
• Authenticated via EXA_API_KEY environment variable; gracefully disabled when the key is not set
• Registered in the tool runtime registry and available in Researcher, OpenCowork, and CheapClaw agent configurations

Usage Example

# Via the tool system (agent-invoked)
tool = ExaSearchTool()
result = tool.execute(task_id, {
    "query": "latest advances in multi-agent systems",
    "max_results": 10,
    "content_mode": "highlights",       # or "text", "summary", "none"
    "category": "research paper",
    "include_domains": ["arxiv.org"],
    "start_published_date": "2025-01-01T00:00:00Z",
    "save_path": "temp/exa_search/results.md"
})

Files Changed

File	Change
tool_server_lite/tools/exa_tools.py	New Exa search tool implementation
tool_server_lite/tools/__init__.py	Added ExaSearchTool import and export
tool_server_lite/registry.py	Registered exa_search in the builtin tool factories
requirements.txt	Added exa-py>=2.0.0
tool_server_lite/requirements.txt	Added exa-py>=2.0.0
config/agent_library/Researcher/level_0_tools.yaml	Added exa_search tool definition
config/agent_library/OpenCowork/level_0_tools.yaml	Added exa_search tool definition
apps/cheapclaw/assets/.../CheapClawWorkerGeneral/level_0_tools.yaml	Added exa_search tool definition
apps/cheapclaw/assets/.../CheapClawSupervisor/level_0_tools.yaml	Added exa_search tool definition
tests/test_exa_tools.py	19 unit tests

Test Plan

• SDK-missing state returns descriptive error
• API-key-missing state returns descriptive error
• Missing query parameter returns error
• Highlights response parsed correctly with metadata (title, URL, author, date)
• Content fallback cascade: highlights -> summary -> text
• Empty content fields handled gracefully
• content_mode=none calls search() instead of search_and_contents()
• Text/summary/highlights modes pass correct SDK parameters
• x-exa-integration: infiagent header is set on every request
• Results saved to file when save_path is provided
• Optional filters (category, domains, text, dates, search_type) passed through correctly
• All 19 tests pass locally with pytest tests/test_exa_tools.py