Add narrow viewport VRT for AnchoredOverlay Fullscreen Variant

[Copilot]

The Fullscreen Variant story is responsive — fullscreen only below 768px — so the default Playwright viewport was only capturing the regular anchored state, not the fullscreen behavior the story is meant to exercise.

Changelog

Changed

• AnchoredOverlay Fullscreen Variant e2e test moved out of the general stories loop into a dedicated block with:
  • Correct button selector ('Open Fullscreen on Narrow' instead of 'Button')
  • Additional narrow viewport test (544px, below the 768px breakpoint) that actually triggers fullscreen overlay behavior

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Test-only change, no shipping artifact affected.

Testing & Reviewing

The narrow viewport test sets viewports['primer.breakpoint.xs'] (544px) before visiting the story, which is below the 768px threshold where variant={{narrow: 'fullscreen', regular: 'anchored'}} switches to fullscreen mode. This produces two snapshot pairs per theme/feature-flag combination: one at default viewport (anchored), one at narrow (fullscreen).

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github-ui (Learn more about how to run integration tests)

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Latest commit: bbf68c9

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