feat: update Exa integration with tracking header and current API params

[tgonzalezc5]

Summary

• Add x-exa-integration: khoj tracking header to both search_with_exa() and read_webpage_with_exa() for API usage attribution
• Replace deprecated numSentences/highlightsPerUrl highlights parameters with maxCharacters per current Exa API docs
• Remove deprecated livecrawl parameter from the contents endpoint (omitting it preserves the same fallback behavior)
• Add unit tests for Exa search response parsing, snippet fallback, header verification, and deprecated param removal

Usage

No configuration changes required. Existing EXA_API_KEY env var continues to work as before.

# Search (unchanged interface, updated internals)
query, results = await search_with_exa("latest AI news", location)
# results["organic"][0] -> {"title": ..., "link": ..., "snippet": ..., "content": ...}

# Webpage reading (unchanged interface)
text = await read_webpage_with_exa("https://example.com", api_key, api_url)

Files changed

• src/khoj/processor/tools/online_search.py -- updated Exa API headers and request params
• tests/test_exa_search.py -- new unit tests for Exa search and webpage reading

Test plan

• Verify x-exa-integration: khoj header is sent in search requests
• Verify x-exa-integration: khoj header is sent in contents requests
• Verify maxCharacters is used instead of deprecated numSentences/highlightsPerUrl
• Verify livecrawl param is no longer sent in contents requests
• Verify search result parsing with highlights, text content, and missing highlights
• Verify empty results and API errors return empty dict gracefully
• Verify location defaults to "US" when country_code is None
• Full test suite requires Django/database setup (CI)