perf: replace JSON.parse(JSON.stringify()) with structuredClone in layout (#22104)

Replace JSON roundtrip with native structuredClone for tool execution result cloning. Also remove unnecessary JSON roundtrip on a static error object literal that is already a fresh value.

src/routes/+layout.svelte

@@ -343,17 +343,11 @@
 			}
 
 			if (cb) {
-				cb(JSON.parse(JSON.stringify(res)));
+				cb(structuredClone(res));
 			}
 		} else {
 			if (cb) {
-				cb(
-					JSON.parse(
-						JSON.stringify({
-							error: 'Tool Server Not Found'
-						})
-					)
-				);
+				cb({ error: 'Tool Server Not Found' });
 			}
 		}
 	};