fix: improve unstate type definition (#9895)

* fix: improve unstate type definition

* tweak

* tweak

.changeset/purple-dragons-peel.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: improve unstate type definition

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

@@ -100,12 +100,12 @@ function unwrap(value, already_unwrapped = new Map()) {
 }
 
 /**
- * @template {StateObject} T
+ * @template T
  * @param {T} value
- * @returns {Record<string | symbol, any>}
+ * @returns {T}
  */
 export function unstate(value) {
-	return unwrap(value);
+	return /** @type {T} */ (unwrap(/** @type {StateObject} */ (value)));
 }
 
 /**