fix: scope SSR boundary failed snippets to their boundary (#18593)

Each `<svelte:boundary>` with:

```svelte
{#snippet failed(error)}
  ...
{/snippet}
```

generated a function named failed. Sibling boundaries placed both
functions in the same SSR scope:

```js
{
  function failed() {}
  function failed() {} // Identifier `failed` has already been declared
}
```

Resulting in a `[PARSE_ERROR] Identifier `failed` has already been
declared` error during build time.

A let declaration such as `{const x = 0}` can create a nested lexical
block, exposing the collision.

The fix gives each boundary its own scope:

```js
{
  function failed() {}
  $$renderer.boundary({ failed }, ...);
}

{
  function failed() {}
  $$renderer.boundary({ failed }, ...);
}
``

.changeset/young-papers-hide.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: scope SSR boundary failed snippets to their boundary

packages/svelte/src/compiler/phases/3-transform/server/visitors/SvelteBoundary.js

@@ -1,4 +1,4 @@
-/** @import { BlockStatement } from 'estree' */
+/** @import { BlockStatement, Statement } from 'estree' */
 /** @import { AST } from '#compiler' */
 /** @import { ComponentContext } from '../types' */
 import * as b from '#compiler/builders';
@@ -84,6 +84,9 @@ export function SvelteBoundary(node, context) {
 	}
 
 	const props = b.object([]);
+	/** @type {Statement[]} */
+	const init = [];
+
 	if (failed_attribute && !failed_snippet) {
 		const failed_callee = build_attribute_value(
 			failed_attribute.value,
@@ -95,13 +98,15 @@ export function SvelteBoundary(node, context) {
 
 		props.properties.push(b.init('failed', failed_callee));
 	} else if (failed_snippet) {
-		context.visit(failed_snippet, context.state);
+		context.visit(failed_snippet, { ...context.state, init });
 		props.properties.push(b.init('failed', failed_snippet.expression));
 	}
 
-	context.state.template.push(
+	const boundary = b.stmt(
-		b.stmt(b.call('$$renderer.boundary', props, b.arrow([b.id('$$renderer')], children_body)))
+		b.call('$$renderer.boundary', props, b.arrow([b.id('$$renderer')], children_body))
 	);
+
+	context.state.template.push(init.length > 0 ? b.block([...init, boundary]) : boundary);
 }
 
 /**

packages/svelte/tests/server-side-rendering/samples/boundary-duplicate-failed-snippets/_expected.html

@@ -0,0 +1 @@
+<div></div>

packages/svelte/tests/server-side-rendering/samples/boundary-duplicate-failed-snippets/main.svelte

@@ -0,0 +1,11 @@
+<div>
+	{const x = 0}
+
+	<svelte:boundary>
+		{#snippet failed()}{/snippet}
+	</svelte:boundary>
+
+	<svelte:boundary>
+		{#snippet failed()}{/snippet}
+	</svelte:boundary>
+</div>