test: add runtime-browser regression test for style directive trailing semicolon

packages/svelte/tests/runtime-browser/samples/style-directive-trailing-semicolon/_config.js

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+import { assert_ok, test } from '../../assert';
+
+export default test({
+	html: `<div style="color: red;"></div>`,
+
+	test({ assert, target, window, component }) {
+		const div = target.querySelector('div');
+		assert_ok(div);
+
+		// Initial value with trailing semicolon — should produce valid CSS
+		assert.equal(window.getComputedStyle(div).color, 'rgb(255, 0, 0)');
+
+		// Update to a new value that also has a trailing semicolon
+		component.color = 'blue;';
+		assert.equal(window.getComputedStyle(div).color, 'rgb(0, 0, 255)');
+
+		// Update to a value without trailing semicolon — should still work
+		component.color = 'green';
+		assert.equal(window.getComputedStyle(div).color, 'rgb(0, 128, 0)');
+	}
+});

packages/svelte/tests/runtime-browser/samples/style-directive-trailing-semicolon/main.svelte

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+<script lang="ts">
+	export let color = 'red;';
+</script>
+
+<div style:color={color}></div>