Fix AnchoredOverlay e2e test button selectors for strict-mode robustness

[Copilot]

Two stories in the AnchoredOverlay VRT suite produced unreliable selectors: Portal Inside Scrolling Element renders 200 buttons named "Button" (Playwright strict-mode violation on .click()), and Fullscreen Variant uses anchor text "Open Fullscreen on Narrow" instead of "Button".

Changelog

Changed

• Added buttonName?: string per-story field to AnchoredOverlay.test.ts (mirrors ActionMenu.test.ts pattern)
• Fullscreen Variant story entry now sets buttonName: 'Open Fullscreen on Narrow' to match its actual anchor label
• Both button locators now use .first() to avoid Playwright strict-mode violations when multiple matching elements are present (e.g. the 200-button grid in Portal Inside Scrolling Element)

New

Removed

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 package release needed.

Testing & Reviewing

Run the VRT suite against AnchoredOverlay stories, specifically verifying:

• Portal Inside Scrolling Element no longer throws a strict-mode violation
• Fullscreen Variant correctly opens the overlay before screenshotting

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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