fix(mcp): improve 401 detection for OAuth flow trigger (#362)

The `StreamableHTTPError` from `@modelcontextprotocol/sdk` stores the
HTTP status in `error.code` (not `error.status`), and some servers
return 'Authentication required' instead of 'Unauthorized' in their
error messages.

This caused the better-chatbot to incorrectly fall back to SSE transport
instead of retrying with OAuth provider when connecting to MCP servers
that require authentication. For example, I found that MCP servers that
were built using the [FastMCP](https://github.com/punkpeye/fastmcp)
Typescript framework did not work with better-chatbot. This PR addresses
that.

Co-authored-by: Håkon Harnes <hakonh@basis.no>

Author: hakonharnes

src/lib/ai/mcp/create-mcp-client.ts

@@ -434,10 +434,12 @@ function isUnauthorized(error: any): boolean {
   return (
     error instanceof UnauthorizedError ||
     error?.status === 401 ||
+    error?.code === 401 ||
     error?.message?.includes("401") ||
     error?.message?.includes("Unauthorized") ||
     error?.message?.includes("invalid_token") ||
-    error?.message?.includes("HTTP 401")
+    error?.message?.includes("HTTP 401") ||
+    error?.message?.includes("Authentication required")
   );
 }