fix: return isError instead of throwing McpError for missing API key

[blackwell-systems]

Summary

Fixes #161. When TAVILY_API_KEY is not set, the CallToolRequestSchema handler throws McpError(ErrorCode.InvalidRequest, ...), which produces a JSON-RPC error frame. MCP clients treat this as a server crash rather than a recoverable failure.

Fix

Replace the throw new McpError(...) with a return of { content: [...], isError: true }. This lets agents read the error message and self-correct (e.g., prompt the user to set the key) instead of seeing an opaque server crash.

This matches the existing error handling pattern already used for Axios API errors at lines 554-573 in the same file.

Before

JSON-RPC error: {"code": -32600, "message": "TAVILY_API_KEY environment variable is required..."}

Agents see a server crash. No recovery path.

After

{
  "content": [{"type": "text", "text": "TAVILY_API_KEY environment variable is required..."}],
  "isError": true
}

Agents see a tool error with a clear message. Can retry or ask the user.

---

Note

Low Risk
Low risk: localized change to request handler error handling that only affects behavior when TAVILY_API_KEY is unset.

Overview
Prevents MCP clients from treating a missing TAVILY_API_KEY as a server crash by replacing the thrown McpError(ErrorCode.InvalidRequest, ...) in the CallToolRequestSchema handler with a normal tool response containing content and isError: true.

^{Reviewed by Cursor Bugbot for commit 586a808. Bugbot is set up for automated code reviews on this repo. Configure here.}