E2E: Improve Playwright test patterns in backfill.spec.ts

[choo121600]

Description

Improve Playwright test patterns in backfill.spec.ts and related page object to align with
Playwright Best Practices.

This PR focuses on improving test patterns only.
It does not change test coverage or behavior.

Checklist

• page.waitForFunction() with DOM queries → locator-based waiting
• page.waitForTimeout() → state-based waiting
• waitForLoadState("networkidle") → wait for specific UI state
• Manual assertions → web-first assertions
• page.evaluate() for DOM manipulation → observe UI state instead
• CSS :has-text() → user-facing locators

Scope

This issue covers improvements in:

• specs/backfill.spec.ts
• pages/BackfillPage.ts

Use case/motivation

No response

Related issues

Part of #63036

Are you willing to submit a PR?

• Yes I am willing to submit a PR!

Code of Conduct

• I agree to follow this project's Code of Conduct

[stats-dev]

Hello! I can take care of this. I'll update the test patterns in backfill.spec.ts. I will open a PR soon. Thank you.