fix(v10/tanstackstart-react): Avoid duplicating Vite config arrays (#24935)

Backport of: #24929

## Differences to the original PR

- Keep the test's existing `@sentry/vite-plugin` import; v10 has not
moved to `@sentry/bundler-plugins/vite`.
- Omit `rolldownOptions` from the full expected config because v10 uses
Vite 6, which does not add that Vite 8 alias. The SDK fix and regression
coverage are otherwise unchanged.

Co-authored-by: GPT-6 <codex@openai.com>

Author: nicohrubec

packages/tanstackstart-react/src/vite/sourceMaps.ts

@@ -100,9 +100,7 @@ export function makeEnableSourceMapsVitePlugin(options: SentryTanstackStartOptio
       enforce: 'post',
       config(viteConfig) {
         return {
-          ...viteConfig,
           build: {
-            ...viteConfig.build,
             sourcemap: getUpdatedSourceMapSettings(viteConfig, options),
           },
         };

packages/tanstackstart-react/test/vite/sourceMaps.test.ts

@@ -1,5 +1,7 @@
+// @vitest-environment node
 import type { SentryVitePluginOptions } from '@sentry/vite-plugin';
 import type { UserConfig } from 'vite';
+import { mergeConfig } from 'vite';
 import { beforeEach, describe, expect, it, vi } from 'vitest';
 import {
   getUpdatedSourceMapSettings,
@@ -34,6 +36,33 @@ describe('makeEnableSourceMapsVitePlugin()', () => {
 
     expect(sourceMapsConfigPlugins).toHaveLength(1);
   });
+
+  it.each([undefined, false, true, 'hidden', 'inline'] as const)(
+    'preserves user config without duplicating arrays when sourcemap is %s',
+    sourcemap => {
+      const plugin = makeEnableSourceMapsVitePlugin({})[0]!;
+      const configHook = plugin.config as (config: UserConfig) => UserConfig;
+      const userConfig: UserConfig = {
+        optimizeDeps: { include: ['react'] },
+        build: {
+          sourcemap,
+          outDir: 'custom-dist',
+          rollupOptions: { plugins: [{ name: 'asset-transform' }] },
+        },
+      };
+
+      const merged = mergeConfig(userConfig, configHook(userConfig));
+
+      expect(merged).toEqual({
+        optimizeDeps: { include: ['react'] },
+        build: {
+          sourcemap: sourcemap ?? 'hidden',
+          outDir: 'custom-dist',
+          rollupOptions: { plugins: [{ name: 'asset-transform' }] },
+        },
+      });
+    },
+  );
 });
 
 describe('makeAddSentryVitePlugin()', () => {