Support dynamic labels for multiple variant ActionMenu

[myabc]

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List the issues that this change affects.

Closes #3826

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• Low risk the change is small, highly observable, and easily rolled back.
• Medium risk changes that are isolated, reduced in scope or could impact few users. The change will not impact library availability.
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• Fixes axe scan violation - This change fixes an existing axe scan violation.
• No new axe scan violation - This change does not introduce any new axe scan violations.
• New axe violation - This change introduces a new axe scan violation. Please describe why the violation cannot be resolved below.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Lookbook)
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

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[changeset-bot]

🦋 Changeset detected

Latest commit: 4de98c3

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

This PR includes changesets to release 1 package

Name	Type
@primer/view-components	Minor

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[Copilot]

Pull request overview

This PR extends the ActionMenu component to support dynamic labels for the multiple select variant. Previously, dynamic labels only worked for single select menus; now they also update dynamically when multiple items are selected in a multi-select menu.

Key changes:

• Extended #setDynamicLabel() method to handle multiple select variant with comma-separated labels
• Added space after the dynamic label prefix for better formatting
• Added test coverage for the new multiple select dynamic label functionality

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
app/components/primer/alpha/action_menu/action_menu_element.ts	Extended dynamic label functionality to support multiple select variant by removing single-select restriction and adding logic to join multiple selected item labels
test/system/alpha/action_menu_test.rb	Renamed existing test for clarity and added new test to verify dynamic labels work correctly for multiple select variant
.changeset/sweet-candies-pay.md	Added changeset entry documenting the new feature

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[Copilot]

The textContent property can return null, which will cause a runtime error when calling .trim() on it. This should be handled to prevent the error. Consider using optional chaining or adding a null check before calling trim.

Suggested change

	.map(label => label.textContent.trim())
	.map(label => label.textContent?.trim() ?? '')

[jonrohan]

Thanks for taking the time on this contribution @myabc but sorry to say we're putting primer view components into maintenance mode. More info is here #3908

As this isn't a fix directly related to a bug I'm going to close.