Prevent virtualized SelectPanel measurement warnings

[tay1orjones]

Closes #8424

Virtualized SelectPanel items were forwarding their measurement ref to both the indexed list item and an unindexed inner element, causing TanStack Virtual to emit a console warning. Route list-semantic refs only to the outer list item so measurement uses the expected element.

Changelog

New

• N/A

Changed

• Prevent virtualized SelectPanel items from logging missing data-index measurement warnings.
• Add regression coverage for ActionList ref routing and virtualized item measurement.

Removed

• N/A

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

Testing & Reviewing

Full build, unit tests, type-check, lint, CSS lint, and formatting checks pass. The VirtualizedBuiltIn Storybook example was also exercised in Chromium and no longer emits the measurement warning while scrolling.

[changeset-bot]

🦋 Changeset detected

Latest commit: 6fff099

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/react	Patch

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[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.

[primer-integration]

Integration test results from github/github-ui PR:

  Changes Passed
  CI Passed
  VRT Passed
  Projects Passed

All checks passed!