fix: avoid tracking SvelteURLSearchParams during URL updates (#18791)

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---


Avoids creating an unintended reactive dependency when
`SvelteURLSearchParams` synchronizes its owning `SvelteURL`.

`#update_url()` now uses `super.toString()` instead of the reactive
`toString()` method. URL synchronization only needs native serialization
and should not subscribe the current reaction to the params version
signal.


Before this change, code such as:

```js
render_effect(() => {
	url.searchParams.set('foo', 'bar');
});
```


could cause the effect to run twice:

1. set() calls #update_url()
2. #update_url() calls reactive toString()
3. toString() subscribes the effect to #version
4. set() increments #version
5. The effect is scheduled again

Signed-off-by: cuishuang <imcusg@gmail.com>

.changeset/fuzzy-urls-rest.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: avoid tracking SvelteURLSearchParams during URL updates

packages/svelte/src/reactivity/url-search-params.js

@@ -42,7 +42,7 @@ export class SvelteURLSearchParams extends URLSearchParams {
 		if (!this.#url || this.#updating) return;
 		this.#updating = true;
 
-		const search = this.toString();
+		const search = super.toString();
 		this.#url.search = search && `?${search}`;
 
 		this.#updating = false;

packages/svelte/src/reactivity/url.test.ts

@@ -115,6 +115,24 @@ test('url.searchParams', () => {
 	cleanup();
 });
 
+test('url.searchParams writes do not create a reactive dependency', () => {
+	const url = new SvelteURL('https://svelte.dev');
+	let runs = 0;
+
+	const cleanup = effect_root(() => {
+		render_effect(() => {
+			runs += 1;
+			url.searchParams.set('foo', 'bar');
+		});
+	});
+
+	flushSync();
+
+	assert.equal(runs, 1);
+
+	cleanup();
+});
+
 test('url.searchParams.set updates url when duplicate values collapse to the same joined string', () => {
 	const url = new SvelteURL('https://svelte.dev?a=ab&a=c');
 	const log: any = [];