fix: remove raw-text hydration markers (#18269)

Fixes #14413.

This keeps the temporary raw-text hydration sentinel used by dynamic
`<svelte:element>` from becoming part of the final DOM. Hydration still
gets a marker to advance through for raw-text children, but the marker
is removed after the child renderer runs, so `<style>` and `<script>`
contents stay identical to SSR output.

Tests:
- `pnpm test hydration -t svelte-head-dynamic-style`
- `pnpm test hydration`
- `pnpm test runtime-runes -t svelte-element`
- `pnpm lint`
- `git diff --check`

---------

Co-authored-by: Puneet Dixit <236133619+puneetdixit200@users.noreply.github.com>
Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/gentle-styles-hydrate.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: remove temporary raw-text hydration markers

packages/svelte/src/internal/client/dom/blocks/svelte-element.js

@@ -88,9 +88,11 @@ export function element(node, get_tag, is_svg, render_fn, get_namespace, locatio
 				assign_nodes(element, element);
 
 				if (render_fn) {
+					var tmp_comment = null;
+
 					if (hydrating && is_raw_text_element(next_tag)) {
-						// prevent hydration glitches
+						// prevent hydration glitches (code just below expects an anchor)
-						element.append(document.createComment(''));
+						element.append((tmp_comment = document.createComment('')));
 					}
 
 					// If hydrating, use the existing ssr comment as the anchor so that the
@@ -114,7 +116,7 @@ export function element(node, get_tag, is_svg, render_fn, get_namespace, locatio
 					// contains children, it's a user error (which is warned on elsewhere)
 					// and the DOM will be silently discarded
 					render_fn(element, child_anchor);
-
+					tmp_comment?.remove();
 					set_animation_effect_override(null);
 				}
 

packages/svelte/tests/hydration/samples/script/_expected.html

@@ -1 +1 @@
-<!--[--><!----><script>{}<!----></script><!----><!--]-->
+<!--[--><!----><script>{}</script><!----><!--]-->

packages/svelte/tests/hydration/samples/svelte-head-dynamic-style/_config.js

@@ -0,0 +1,7 @@
+import { test } from '../../test';
+
+export default test({
+	props: {
+		css: 'body { color: red; }'
+	}
+});

packages/svelte/tests/hydration/samples/svelte-head-dynamic-style/main.svelte

@@ -0,0 +1,9 @@
+<script>
+	let { css } = $props();
+</script>
+
+<svelte:head>
+	<svelte:element this="style" type="text/css">{css}</svelte:element>
+</svelte:head>
+
+<p>content</p>