Improve Playwright test patterns in dag-code-tab.spec.ts #63410

[haseebmalik18]

Improve Playwright test patterns in DagCodePage.ts to align with Playwright best practices.

• Replace .waitFor() with expect().toBeVisible() web-first assertions
• Replace manual count assertions with expect().not.toHaveCount(0)
• Replace textContent() and manual assert with expect().toHaveText()
• Replace getAttribute() and manual regex with expect().toHaveAttribute()
• Replace one-shot evaluate() and manual assert with expect.poll() retrying assertion
• Move inline viewLines locator to class property

closes: #63410

[choo121600]

Looks good to me
Cool :)