MCP servers deployed on AWS Lambda Function URLs stopped working after upgrading from v1.24.2 to v1.25.0+

[severi]

1.25.0: Connection fails
1.25.3: Connection works but tool calls return null

(didn't test 1.25.1-1.25.2)

Suspect PR #1209's fetch transport doesn't play well with Lambda + serverless-express which expects Node.js-style streaming.

To Reproduce

1. Deploy MCP server to Lambda Function URL with StreamableHTTPServerTransport + @codegenie/serverless-express
2. Connect with Claude Code
3. Call any tool

Expected behavior

Tool calls return results (works fine on 1.24.2).

Logs

1.25.0:
Failed to reconnect to test-mcp-server.

1.25.3:
{"code":"invalid_union","errors":[[{"expected":"object","code":"invalid_type","path":[],"message":"Invalid input: expected object, received null"}]],"path":[],"message":"Invalid input"}