Improve Playwright test patterns in dag-tasks.spec.ts

Replace anti-patterns in DagsPage.ts and dag-tasks.spec.ts with
Playwright best practices:

• Replace CSS :has-text() selectors with getByRole() user-facing locators
• Replace page.locator('[data-testid="..."]') with page.getByTestId("...")
• Replace page.waitForTimeout() with state-based waits
• Replace locator("a") with getByRole("link")
• Replace locator('div[role="listbox/option"]') with getByRole()
• Replace .waitFor({ state: "visible" }) with expect().toBeVisible()
• Replace .dispatchEvent("click") with .click()
• Replace locator("table") / locator("th") with getByRole("table") / getByRole("columnheader")

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

Part of #63036.
closes: #63425

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