fix(tanstackstart-react): Move Sentry.init() to client entry point

Moves client-side Sentry.init() from getRouter() to client.tsx so it
runs before hydration. The browser tracing integration is added later
via addIntegration() once the router is available.

This ensures errors and breadcrumbs from code that runs before
hydration (e.g. third-party library inits) are captured.

Fixes #21088

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: nicohrubec

dev-packages/e2e-tests/test-applications/tanstackstart-react-cloudflare/src/client.tsx

@@ -1,7 +1,16 @@
+import * as Sentry from '@sentry/browser';
 import { StartClient } from '@tanstack/react-start/client';
 import { StrictMode, startTransition } from 'react';
 import { hydrateRoot } from 'react-dom/client';
 
+Sentry.init({
+  environment: 'qa',
+  dsn: 'https://public@dsn.ingest.sentry.io/1337',
+  tracesSampleRate: 1.0,
+  release: 'e2e-test',
+  tunnel: 'http://localhost:3031/',
+});
+
 console.log('early-breadcrumb-from-client-entry');
 
 if (window.location.pathname === '/crash-before-hydration') {

dev-packages/e2e-tests/test-applications/tanstackstart-react-cloudflare/src/router.tsx

@@ -9,14 +9,7 @@ export const getRouter = () => {
   });
 
   if (!router.isServer) {
-    Sentry.init({
-      environment: 'qa',
-      dsn: 'https://public@dsn.ingest.sentry.io/1337',
-      integrations: [Sentry.browserTracingIntegration()],
-      tracesSampleRate: 1.0,
-      release: 'e2e-test',
-      tunnel: 'http://localhost:3031/',
-    });
+    Sentry.addIntegration(Sentry.browserTracingIntegration());
   }
 
   return router;

dev-packages/e2e-tests/test-applications/tanstackstart-react/src/client.tsx

@@ -1,7 +1,18 @@
+import * as Sentry from '@sentry/tanstackstart-react';
 import { StartClient } from '@tanstack/react-start/client';
 import { StrictMode, startTransition } from 'react';
 import { hydrateRoot } from 'react-dom/client';
 
+Sentry.init({
+  environment: 'qa', // dynamic sampling bias to keep transactions
+  dsn: __APP_DSN__,
+  // We recommend adjusting this value in production, or using tracesSampler
+  // for finer control
+  tracesSampleRate: 1.0,
+  release: 'e2e-test',
+  tunnel: __APP_TUNNEL__,
+});
+
 console.log('early-breadcrumb-from-client-entry');
 
 if (window.location.pathname === '/crash-before-hydration') {

dev-packages/e2e-tests/test-applications/tanstackstart-react/src/router.tsx

@@ -9,16 +9,7 @@ export const getRouter = () => {
   });
 
   if (!router.isServer) {
-    Sentry.init({
-      environment: 'qa', // dynamic sampling bias to keep transactions
-      dsn: __APP_DSN__,
-      integrations: [Sentry.tanstackRouterBrowserTracingIntegration(router)],
-      // We recommend adjusting this value in production, or using tracesSampler
-      // for finer control
-      tracesSampleRate: 1.0,
-      release: 'e2e-test',
-      tunnel: __APP_TUNNEL__,
-    });
+    Sentry.addIntegration(Sentry.tanstackRouterBrowserTracingIntegration(router));
   }
 
   return router;