feat: add Exa AI-powered search tool

[Teo (tgonzalezc5)]

Summary

Adds Exa as a selectable search provider alongside Tavily and the native OpenAI/Anthropic web-search tools.

• New SearchAPI.EXA enum value plus a UI option in Configuration.search_api.
• exa_search tool and exa_search_async wrapper in utils.py, mirroring the Tavily shape so the existing dedup-and-summarize pipeline applies unchanged.
• Exposes Exa's key features as injected tool args: search_type (auto/neural/fast), category, include_domains / exclude_domains, and start_published_date / end_published_date.
• Uses the official exa-py SDK; sync calls are dispatched via asyncio.to_thread so the event loop stays free, and queries run in parallel via asyncio.gather.
• Reads EXA_API_KEY from the environment or from configurable.apiKeys when GET_API_KEYS_FROM_CONFIG=true, matching the existing Tavily pattern.
• Bumps exa-py floor to >=2.0.0 and adds EXA_API_KEY= to .env.example.

Usage

from open_deep_research.configuration import Configuration, SearchAPI

config = Configuration(search_api=SearchAPI.EXA)
# or via env: EXA_API_KEY=... and selecting "Exa" in the LangGraph Studio UI

The agent will then call exa_search with a list of queries; the helper retrieves results with text + AI summary + highlights and feeds them into the same summarization step the Tavily path uses.

Files changed

• src/open_deep_research/configuration.py — register SearchAPI.EXA + UI option.
• src/open_deep_research/utils.py — exa_search tool, exa_search_async, _format_exa_response, get_exa_api_key, dispatch in get_search_tool.
• pyproject.toml — exa-py>=2.0.0.
• .env.example — add EXA_API_KEY=.
• tests/test_exa_search.py — 15 unit tests.

Test plan

• python -m ruff check src/open_deep_research/utils.py tests/test_exa_search.py — no new findings.
• python -m pytest tests/test_exa_search.py -v --asyncio-mode=auto — 15 passed:
  • response normalization for typed and dict result objects
  • content / raw_content fallbacks (text-only, summary-only, highlights-only, none)
  • URL deduplication and skipping of result entries with no URL
  • get_exa_api_key env vs. apiKeys config lookup
  • exa_search_async raises when key is missing or domain filters conflict
  • SDK invocation parameters and x-exa-integration tracking header
  • get_search_tool(SearchAPI.EXA) returns the wired Exa tool
• End-to-end run against the LangGraph Studio UI with search_api=exa (requires a live EXA_API_KEY; not exercised here).