fix(sveltekit): Fix file system race condition in source map cleaning (#19714)

Replace `existsSync` guard with try/catch around read+write operations
to eliminate the time-of-check to time-of-use race condition (CWE-367)
flagged by CodeQL (code-scanning alert #439).

closes
https://github.com/getsentry/sentry-javascript/security/code-scanning/439

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: chargome

packages/sveltekit/src/vite/sourceMaps.ts

@@ -213,14 +213,16 @@ export async function makeCustomSentryVitePlugins(
         // We need to remove the query string from the source map files that our auto-instrument plugin added
         // to proxy the load functions during building.
         const mapFile = `${file}.map`;
-        if (fs.existsSync(mapFile)) {
+        try {
           const mapContent = (await fs.promises.readFile(mapFile, 'utf-8')).toString();
           const cleanedMapContent = mapContent.replace(
             // oxlint-disable-next-line sdk/no-regexp-constructor -- no user input + escaped anyway
             new RegExp(escapeStringForRegex(WRAPPED_MODULE_SUFFIX), 'gm'),
             '',
           );
           await fs.promises.writeFile(mapFile, cleanedMapContent);
+        } catch {
+          // Map file doesn't exist, nothing to clean
         }
       }