Request contents.summary from Exa /search API and combine up to
3 non-empty summaries into SearchResponse.answer, replacing the
previous 'No answer text returned' output.
Changes:
- Add contents.summary to every Exa search request body
- Add summary field to ExaSearchResult interface
- Export synthesizeAnswer() to build combined answer from summaries
- Export buildExaRequestBody() for testability
- Export normalizeSearchType() (was private)
- Prefer summary over text/highlights for snippet field (falsy check)
- Only synthesize answer from results that have a URL (mirrors sources filter)
Tests: 37 new tests covering normalizeSearchType, buildExaRequestBody,
synthesizeAnswer (pure unit tests) and searchExa (mocked fetch)
