fix: coerce symbols to empty string in set_text

Under experimental.async, $bindable can surface Symbol(UNINITIALIZED).
set_text previously passed symbols through and threw when setting nodeValue.

.changeset/set-text-symbol-guard.md

@@ -0,0 +1,9 @@
+---
+'svelte': patch
+---
+
+fix: coerce symbols to empty string in `set_text`
+
+Under `experimental.async`, `$bindable` props can briefly surface `Symbol(UNINITIALIZED)`.
+`set_text` treated symbols as already-stringifiable values, then threw when assigning
+`text.nodeValue = \`${str}\``. Treat symbols like nullish values so text nodes stay stable.

packages/svelte/src/internal/client/render.js

@@ -45,7 +45,7 @@ export function set_should_intro(value) {
  */
 export function set_text(text, value) {
 	// For objects, we apply string coercion (which might make things like $state array references in the template reactive) before diffing
-	var str = value == null ? '' : typeof value === 'object' ? `${value}` : value;
+	var str = value == null || typeof value === 'symbol' ? '' : typeof value === 'object' ? `${value}` : value;
 	// prettier-ignore
 	if (str !== (/** @type {any} */ (text)[TEXT_CACHE] ??= text.nodeValue)) {
 		/** @type {any} */ (text)[TEXT_CACHE] = str;