fix: reapply context after transforming error during SSR (#18099)

Don't have a test for it and no bug report but I stumbled upon this and
I'm very certain not restoring context here is wrong since it means the
failed snippet rendering gets the wrong context.

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/cruel-boxes-serve.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: reapply context after transforming error during SSR

packages/svelte/src/internal/server/renderer.js

@@ -715,7 +715,12 @@ export class Renderer {
 						const { context, failed, transformError } = item.#boundary;
 
 						set_ssr_context(context);
-						let transformed = await transformError(error);
+
+						let promise = transformError(error);
+						set_ssr_context(null);
+
+						let transformed = await promise;
+						set_ssr_context(context);
 
 						// Render the failed snippet instead of the partial children content
 						const failed_renderer = new Renderer(item.global, item);