[Server] Connections to Github Copilot CLI time out on subscriptions/listen #516

Describe the bug
Github Copilot CLI (version 1.0.85 and above) fails to connect to stateless MCP server. Authentication is successful but the subsequent subscriptions/listen request never resolves, instead timing out after 10000 ms.
Adding @obflush before the flush on line 83 of StatelessResponder resolves this issue.

To Reproduce
Steps to reproduce the behavior:

1. Run a stateless MCP server with default settings
2. Add an MCP server using Github Copilot CLI version ^1.0.85
3. Authenticate the server
4. The server will successfully authenticate but the connection will fail with a timeout error

Expected behavior
The server will successfully authenticate and connect