fix(router-core): qualify pagehide event listener

[MoonBrillante]

Summary

Qualify the pagehide listener registration with window.addEventListener.

This ensures the correct receiver is used when EventTarget.prototype.addEventListener has been patched by instrumentation libraries.

A regression assertion was added to verify that the pagehide listener is registered with window as its context.

Fixes #8024

Tests

pnpm nx run @tanstack/router-core:test:unit -- tests/scroll-restoration.test.ts

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

## Summary by CodeRabbit

- **Bug Fixes**
  - Improved scroll restoration reliability by ensuring page navigation events are registered on the browser window.
- **Tests**
  - Added coverage confirming the scroll restoration listener is attached to the correct browser target.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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Reviewing files that changed from the base of the PR and between af8dcb8 and f20e66a.

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• packages/router-core/src/scroll-restoration.ts
• packages/router-core/tests/scroll-restoration.test.ts

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