feat: add Exa AI-powered search tool

[tgonzalezc5]

Summary

• Adds ExaProvider implementing the existing SearchProvider ABC, alongside DuckDuckGoProvider and TavilyProvider.
• Exa is a neural/semantic web search API that returns relevance-ranked results along with rich content (text, highlights, summary) in a single search_and_contents call, so the provider can populate SearchResult.snippet without a separate content-extraction round-trip.
• Selection is config-driven via SEARCH_PROVIDER=exa, with EXA_API_KEY, EXA_SEARCH_TYPE, and EXA_CATEGORY for tuning. Everything else (domain/text filters, date ranges, content sizes) is exposed via ExaProvider constructor kwargs for programmatic use.

Usage

# .env
SEARCH_PROVIDER=exa
EXA_API_KEY=your-exa-api-key
EXA_SEARCH_TYPE=auto        # optional: auto | neural | fast | deep
EXA_CATEGORY=research paper # optional

from src.utils.web_utils import ExaProvider, WebSearchTool

provider = ExaProvider(
    api_key="...",
    max_results=5,
    search_type="neural",
    category="research paper",
    include_domains=["arxiv.org"],
    start_published_date="2025-01-01",
)
results = await WebSearchTool(providers=[provider]).search_async("llm reasoning benchmarks")

Implementation notes

• Follows the existing TavilyProvider pattern for circuit breaker, rate-limit detection, and error mapping (RateLimitError / SearchError).
• exa-py's default client is synchronous, so the call is offloaded with asyncio.to_thread (same approach as DuckDuckGoProvider._execute_search).
• Snippet extraction cascades summary → joined highlights → trimmed text → empty string, so any combination of returned content fields works.

Files changed

• src/utils/web_utils.py — new ExaProvider class
• src/utils/tools.py — register Exa in _build_search_providers
• src/config.py — EXA_API_KEY, EXA_SEARCH_TYPE, EXA_CATEGORY fields
• requirements.txt, pyproject.toml — add exa-py>=2.0.0
• tests/test_exa_provider.py — 12 unit tests (parsing, request kwargs, snippet fallback, error mapping, provider registration); no live API calls

Test plan

• pytest tests/test_exa_provider.py — 12/12 passing locally
• ruff check introduces no new warnings on touched files
• Smoke test with a real EXA_API_KEY in the running agent