Proxy servers like Nginx buffer responses by default, which causes
SSE streams to be delayed. Adding the X-Accel-Buffering: no header
instructs reverse proxies to disable response buffering for SSE
endpoints, ensuring real-time streaming delivery.

Applied to all three SSE response locations:
- HttpServletSseServerTransportProvider (SSE endpoint)
- HttpServletStreamableServerTransportProvider (GET and POST SSE)

Fixes modelcontextprotocol#293

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>