fix: bypass error boundaries for hydration recovery (#18841)

Fixes #18840

This PR rethrows the hydration error rather than catching it at the
boundaries so that the svelte hydration warning can surface

### Before submitting the PR, please make sure you do the following

- [x] It's really useful if your PR references an issue where it is
discussed ahead of time. In many cases, features are absent for a
reason. For large changes, please create an RFC:
https://github.com/sveltejs/rfcs
- [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
- [x] This message body should clearly illustrate what problems it
solves.
- [x] Ideally, include a test that fails without this PR but passes with
it.
- [x] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).

### Tests and linting

- [x] Run the tests with `pnpm test` and lint the project with `pnpm
lint`

---------

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

.changeset/tidy-pandas-hydrate.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: prevent hydration mismatch recovery from being intercepted by error boundaries

packages/svelte/src/internal/client/dom/blocks/boundary.js

@@ -1,6 +1,10 @@
 /** @import { Effect, Source, TemplateNode, } from '#client' */
 import { BOUNDARY_EFFECT, EFFECT_PRESERVED, EFFECT_TRANSPARENT } from '#client/constants';
-import { HYDRATION_START_ELSE, HYDRATION_START_FAILED } from '../../../../constants.js';
+import {
+	HYDRATION_ERROR,
+	HYDRATION_START_ELSE,
+	HYDRATION_START_FAILED
+} from '../../../../constants.js';
 import { component_context, set_component_context } from '../../context.js';
 import { invoke_error_boundary } from '../../error-handling.js';
 import {
@@ -445,6 +449,10 @@ export class Boundary {
 
 	/** @param {unknown} error */
 	error(error) {
+		if (error === HYDRATION_ERROR) {
+			throw error;
+		}
+
 		// If we have nothing to capture the error, or if we hit an error while
 		// rendering the fallback, re-throw for another boundary to handle
 		if (!this.#props.onerror && !this.#props.failed) {

packages/svelte/src/internal/client/error-handling.js

@@ -1,7 +1,7 @@
 /** @import { Derived, Effect } from '#client' */
 /** @import { Boundary } from './dom/blocks/boundary.js' */
 import { DEV } from 'esm-env';
-import { FILENAME } from '../../constants.js';
+import { FILENAME, HYDRATION_ERROR } from '../../constants.js';
 import { is_firefox } from './dom/operations.js';
 import {
 	ERROR_VALUE,
@@ -52,6 +52,10 @@ export function handle_error(error) {
  * @param {Effect | null} effect
  */
 export function invoke_error_boundary(error, effect) {
+	if (error === HYDRATION_ERROR) {
+		throw error;
+	}
+
 	if (effect !== null && (effect.f & DESTROYED) !== 0) {
 		return;
 	}

packages/svelte/tests/hydration/samples/boundary-mismatch-recovery/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	expect_hydration_error: true
+});

packages/svelte/tests/hydration/samples/boundary-mismatch-recovery/_expected.html

@@ -0,0 +1 @@
+<!----><p><p>Valid HTML fragment</p><!----></p><!---->

packages/svelte/tests/hydration/samples/boundary-mismatch-recovery/main.svelte

@@ -0,0 +1,7 @@
+<svelte:boundary>
+	<p>{@html '<p>Valid HTML fragment</p>'}</p>
+
+	{#snippet failed()}
+		<p>boundary fallback</p>
+	{/snippet}
+</svelte:boundary>