fix: sync SvelteURL port when protocol setter clears it (#18705)

Per the WHATWG URL spec, assigning a new protocol can clear the URL's port when the current port equals the new scheme's default port. Therefore also `set` the port when the protocol is updated.

.changeset/svelte-url-protocol-port.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: sync `SvelteURL` port signal when the protocol setter clears the port

packages/svelte/src/reactivity/url.js

@@ -163,6 +163,8 @@ export class SvelteURL extends URL {
 	set protocol(value) {
 		super.protocol = value;
 		set(this.#protocol, super.protocol);
+		// changing the protocol can clear the port when it matches the new scheme's default
+		set(this.#port, super.port);
 	}
 
 	get search() {

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

@@ -240,3 +240,25 @@ test('url.searchParams.forEach re-runs when the search string changes via the UR
 
 	cleanup();
 });
+
+test('url.port is updated when the protocol change clears the port', () => {
+	const url = new SvelteURL('http://example.com:443/');
+	const log: any = [];
+
+	const cleanup = effect_root(() => {
+		render_effect(() => {
+			log.push(url.port);
+		});
+	});
+
+	flushSync(() => {
+		// 443 is the default port for https, so it gets stripped
+		url.protocol = 'https:';
+	});
+
+	assert.equal(url.port, '');
+	assert.equal(url.href, 'https://example.com/');
+	assert.deepEqual(log, ['443', '']);
+
+	cleanup();
+});