test(tanstackstart): Deterministically match the server-side error event (#22137)

The tanstackstart server-function error test flaked: it asserted the
error's mechanism is `auto.middleware.tanstackstart.server_function`,
but occasionally received
`auto.browser.global_handlers.onunhandledrejection` instead.

_Root cause_: the thrown error propagates back to the client over the
server-function RPC and is captured a second time on the client as an
`onunhandledrejection` with the **same message**. The `waitForError`
predicate matched on message only, so it raced the two events and
sometimes resolved with the client-side duplicate.

The predicate now also matches the server mechanism, so the correct
server-side event is always selected. The same latent race exists in the
API-route test (identical message-only predicate + specific-mechanism
assertion), so that predicate is hardened the same way.

Fixes #21912

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: mydea

dev-packages/e2e-tests/test-applications/tanstackstart-react/tests/errors.test.ts

@@ -43,7 +43,13 @@ test('Sends client-side error to Sentry with auto-instrumentation', async ({ pag
 
 test('Sends server-side function error to Sentry with auto-instrumentation', async ({ page }) => {
   const errorEventPromise = waitForError('tanstackstart-react', errorEvent => {
-    return errorEvent?.exception?.values?.[0]?.value === 'Sentry Server Function Test Error';
+    // The thrown error propagates back to the client over the server-function RPC and is also
+    // captured there as an `onunhandledrejection` with the same message. Match on the server-function
+    // mechanism so we deterministically pick the server-side event instead of racing the client one.
+    return (
+      errorEvent?.exception?.values?.[0]?.value === 'Sentry Server Function Test Error' &&
+      errorEvent?.exception?.values?.[0]?.mechanism?.type === 'auto.middleware.tanstackstart.server_function'
+    );
   });
 
   await page.goto(`/`);
@@ -74,7 +80,12 @@ test('Sends server-side function error to Sentry with auto-instrumentation', asy
 
 test('Sends API route error to Sentry with auto-instrumentation', async ({ page }) => {
   const errorEventPromise = waitForError('tanstackstart-react', errorEvent => {
-    return errorEvent?.exception?.values?.[0]?.value === 'Sentry API Route Test Error';
+    // As with the server-function test, guard against a same-message client-side duplicate by
+    // matching the server request mechanism, so we always assert against the server-side event.
+    return (
+      errorEvent?.exception?.values?.[0]?.value === 'Sentry API Route Test Error' &&
+      errorEvent?.exception?.values?.[0]?.mechanism?.type === 'auto.middleware.tanstackstart.request'
+    );
   });
 
   await page.goto(`/`);