Refactor Playwright tests in dag-calendar-tab (#63409)

[snesan821]

Description

This PR addresses Playwright testing anti-patterns outlined in Issue #63409, specifically focusing on the DagCalendarTab page object.

Changes:

• Removed deprecated page.waitForFunction() DOM query used to count calendar cells.
• Implemented web-first assertion (expect(locator).toBeVisible()) utilizing existing calendarCells locator to ensure the calendar grid is fully loaded and stable.

closes: #63409

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Was generative AI tooling used to co-author this PR?

• Yes (Kiro Agentic IDE)

Generated-by: Kiro following the guidelines

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[boring-cyborg]

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