fix: suppress SSE client-disconnect noise in GlobalExceptionHandler

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).

trueref-adapters/src/main/java/com/trueref/adapter/in/rest/GlobalExceptionHandler.java

@@ -18,8 +18,9 @@ import org.springframework.validation.FieldError;
 import org.springframework.web.bind.MethodArgumentNotValidException;
 import org.springframework.web.bind.annotation.ExceptionHandler;
 import org.springframework.web.bind.annotation.RestControllerAdvice;
-import org.springframework.web.server.ResponseStatusException;
 import org.springframework.web.servlet.resource.NoResourceFoundException;
+import org.springframework.web.context.request.async.AsyncRequestNotUsableException;
+import org.springframework.web.server.ResponseStatusException;
 
 /** Central translator from domain / validation exceptions to HTTP + {@link ErrorResponse} JSON. */
 @RestControllerAdvice
@@ -98,6 +99,14 @@ public class GlobalExceptionHandler {
         return status(HttpStatus.INTERNAL_SERVER_ERROR, ex);
     }
 
+    // Client disconnected from an SSE stream before the server finished writing.
+    // This is normal (browser tab closed, MCP client reconnected, etc.) — log at DEBUG only
+    // and return nothing; the response socket is already gone.
+    @ExceptionHandler(AsyncRequestNotUsableException.class)
+    public void handleClientDisconnect(AsyncRequestNotUsableException ex) {
+        log.debug("SSE client disconnected: {}", ex.getMessage());
+    }
+
     @ExceptionHandler(Exception.class)
     public ResponseEntity<ErrorResponse> handleUnexpected(Exception ex) {
         log.error("unexpected error", ex);