fix(block_import): log engine errors in new_payload instead of silently dropping

Previously, when engine.new_payload() returned an Err (e.g. EngineUnavailable),
the error was silently discarded with no log, making it impossible to diagnose
broken engine integration from block import logs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: chee-chyuan

src/node/network/block_import/service.rs

@@ -326,7 +326,16 @@ where
                     }
                     _ => None,
                 },
-                Err(err) => None,
+                Err(err) => {
+                    tracing::warn!(
+                        target: "bsc::block_import",
+                        block_hash = %block_hash,
+                        block_number = header.number,
+                        %err,
+                        "Engine unavailable or failed to process new payload"
+                    );
+                    None
+                }
             }
         })
     }