langchain-exa: update default search type from neural to auto

[theishangoswami]

The Exa retriever currently defaults to neural search type, but auto is now the recommended default. The auto type lets the Exa API pick the best search strategy for each query.

Changes:

• Update default type in ExaSearchRetriever from neural to auto
• Reorder the type Literal in ExaSearchResults tool to put auto first
• Small description update to mention Exa is a web search API built for AI

These are small changes and I can open a PR if this gets approved.

[vivek12345]

Ishan Goswami (@theishangoswami) are you making a PR for this. In case you aren't I would be happy to do that