Fix ActionMenu mnemonic handling

[scotttesler]

• Closes ActionMenu opens on modified or unmatched mnemonic keypresses #4167

Prevents modified and unmatched mnemonic keypresses from opening a closed ActionMenu.

What are you trying to accomplish?

Ignore Control-, Command-, and Alt-modified printable keypresses during mnemonic handling, and leave a closed menu unchanged when no item mnemonic matches. Unmodified matching mnemonics continue to open the menu and focus the matching item.

Integration

No production integration updates are required.

List the issues that this change affects.

• ActionMenu opens on modified or unmatched mnemonic keypresses #4167

Risk Assessment

• Low risk the change is small, highly observable, and easily rolled back. The behavior change is limited to printable-key mnemonic handling and has focused system coverage.
• Medium risk changes that are isolated, reduced in scope or could impact few users. The change will not impact library availability.
• High risk changes are those that could impact customers and SLOs, low or no test coverage, low observability, or slow to rollback.

What approach did you choose and why?

Check the keyboard event's modifier flags before mnemonic lookup, while intentionally leaving Shift available for case-insensitive mnemonic matching. If lookup produces no matching item, return before popover or focus handling instead of allowing a negative index to select the final item.

This keeps the change at the shared mnemonic behavior boundary without altering arrow-key navigation or matching mnemonic behavior.

Anything you want to highlight for special attention from reviewers?

focus-group is shared by ActionMenu and SelectPanel, so the modifier and unmatched-key guards apply consistently to both components.

Verification

• npx eslint app/components/primer/focus_group.ts
• npx tsc --noEmit
• bundle exec rubocop test/system/alpha/action_menu_test.rb
• Focused Chrome system tests for modified, unmatched, and matching mnemonics

Accessibility

• No new axe scan violation - This change does not introduce any new axe scan violations.

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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Latest commit: 0c6916f

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