Add Serply as an alternative web search provider

What

Adds Serply as an alternative provider for the web_search tool. If TAVILY_API_KEY is not set but SERPLY_API_KEY is, the tool uses Serply instead of returning an error.

Why

web_search currently hard-requires Tavily. Serply is a SERP API (Google/Bing, plus Scholar and other verticals) that passes the query straight through as Google search syntax, so dork operators (site:, filetype:, intitle:) work unmodified, which is useful for recon in a pentest agent. Existing behavior with Tavily is unchanged, and the tool still works with neither key set (same error path, just mentioning both options).

Notes

• Response fields normalized to the same shape _format_results already expects from Tavily, so no duplicate formatting logic.
• Serply sits behind Cloudflare and rejects the default httpx User-Agent, so the request sends an explicit one.

Testing

Ran web_search directly with SERPLY_API_KEY set and TAVILY_API_KEY unset, using a dork-style query (site:github.com filetype:env ...), and got back real results with titles and links in the same format as the existing Tavily output. Also confirmed the tool still returns the original error message when neither key is set.

Disclosure: I work with Serply. Happy to adjust scope, naming, or drop this entirely if it isn't a direction you want for the project.