fix: handle BrokenPipeError in LMHandler during parallel execution

When running parallel evaluations, multiple workers connect to the LMHandler
socket server. When workers complete and close their sockets, BrokenPipeError
was raised during response transmission, causing noisy error output.

Changes:
- Add _safe_send() helper that catches connection errors silently
- Handle BrokenPipeError, ConnectionError, ConnectionResetError, and OSError
  separately from other exceptions
- Use _safe_send() for all socket_send() calls in handle() method

This prevents cascading errors where the exception handler tried to send
an error response on an already-broken socket.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: alexzhang13

rlm/core/lm_handler.py

@@ -22,7 +22,7 @@ def handle(self):
             request_data = socket_recv(self.connection)
             if not isinstance(request_data, dict):
                 response = LMResponse.error_response("Request must be a JSON object")
-                socket_send(self.connection, response.to_dict())
+                self._safe_send(response)
                 return
 
             request = LMRequest.from_dict(request_data)
@@ -37,11 +37,26 @@ def handle(self):
             else:
                 response = LMResponse.error_response("Missing 'prompt' or 'prompts' in request.")
 
-            socket_send(self.connection, response.to_dict())
+            self._safe_send(response)
+
+        except (BrokenPipeError, ConnectionError, ConnectionResetError, OSError) as e:
+            # Client disconnected - this is expected during parallel execution
+            # when workers complete and close their sockets. Silently ignore.
+            pass
 
         except Exception as e:
+            # Try to send error response, but don't fail if socket is broken
             response = LMResponse.error_response(str(e))
+            self._safe_send(response)
+
+    def _safe_send(self, response: LMResponse) -> bool:
+        """Send response, returning False if the socket is broken."""
+        try:
             socket_send(self.connection, response.to_dict())
+            return True
+        except (BrokenPipeError, ConnectionError, ConnectionResetError, OSError):
+            # Client disconnected - silently ignore
+            return False
 
     def _handle_single(self, request: LMRequest, handler: "LMHandler") -> LMResponse:
         """Handle a single prompt request."""