fix(AnchoredOverlay): prevent ref overwrite from overlayProps spread

[Copilot]

Closes https://github.com/github/primer/issues/6090

In React 19, forwardRef usage results in overlayProps.ref being null more frequently. When spreading overlayProps after the ref prop, this null value overwrites the internal updateOverlayRef, breaking overlay positioning.

Changelog

Changed

• AnchoredOverlay now merges overlayProps.ref with internal ref using custom callback instead of allowing prop spread to overwrite
• Added assignRef utility to handle both callback refs and ref objects

Testing

Added test coverage for:

• Ref objects passed via overlayProps.ref
• Callback refs passed via overlayProps.ref

Rollout strategy

• Patch release

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 (Learn more about how to run integration tests)

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Latest commit: 51bbd60

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