Add markdown output and travel tools

[adarshdigievo]

Summary

• Add dedicated Hotels, Flights, and Travel Explore tools.
• Return SerpApi Markdown by default from direct tools while preserving the output: "json" override.
• Apply result limits to Markdown responses for Maps, News, and Shopping without removing frontmatter, links, or pagination.
• Validate travel dates, airport codes and KGMIDs, passenger counts, children’s ages, hotel filters, and round-trip departure tokens.