fix: don't override instance methods during legacy class creation (#10834)

fixes #10805

Author: dummdidumm

.changeset/dull-pots-add.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: don't override instance methods during legacy class creation

packages/svelte/src/legacy/legacy-client.js

@@ -79,6 +79,7 @@ class Svelte4Component {
 		});
 
 		for (const key of Object.keys(this.#instance)) {
+			if (key === '$set' || key === '$destroy' || key === '$on') continue;
 			define_property(this, key, {
 				get() {
 					return this.#instance[key];