AsyncRequestNotUsableException is thrown when the SSE client disconnects
before the server finishes writing (normal: browser tab close, MCP
client reconnect). The catch-all handler was logging it at ERROR level
and then attempting to write a JSON ErrorResponse onto a text/event-stream
response that no longer had a converter, producing a second spurious
HttpMessageNotWritableException log entry.
Fix: add a dedicated @ExceptionHandler(AsyncRequestNotUsableException)
that logs at DEBUG only and returns void (no body).
cfb35b35c0