fix(editor): cross browser test stability (#14897)

#### PR Dependency Tree


* **PR #14897** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* Improved reliability of shape and connector detection by forcing full
DOM renders during waits.
* Fixed race conditions in code-block theme loading and cleanup when
components unmount.
* Refined viewport element discovery to correctly handle
rotated/canvas-layer elements and avoid stale DOM removal.

* **Tests**
  * Increased polling timeouts and retries to reduce flakiness.
* Disabled per-file parallelism and ensured test setup performs full
cleanup before starting; extended test timeout.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

blocksuite/integration-test/vitest.config.ts

@@ -16,6 +16,7 @@ export default defineConfig(_configEnv =>
     plugins: [vanillaExtractPlugin()],
     test: {
       include: ['src/__tests__/**/*.spec.ts'],
+      fileParallelism: false,
       retry: process.env.CI === 'true' ? 3 : 0,
       browser: {
         enabled: true,