The Exa MCP endpoint at https://mcp.exa.ai/mcp is publicly accessible
without an API key. Previously, all Exa tools bailed early with
'EXA_API_KEY not found' when no key was set, even though the basic
tools (web_search_exa, company_research_exa, get_code_context_exa)
work fine unauthenticated.
Changes:
- fetchExaTools/callExaTool: accept null apiKey, omit exaApiKey param
- MCPWrappedTool.execute: only require key for Websets tools
- createExaTool (factory): remove early-return guard on missing key
Fixes#151
