fix(sharing-dialog): add keyboard navigation to autocomplete

[ericcowan]

Summary

• allow ArrowDown from the SharingDialog autocomplete input to move focus into the results menu
• reuse the menu's existing arrow-key navigation and Enter selection behavior
• add regression coverage for moving into the menu, navigating down/up, and selecting a result

Why

The User or group autocomplete exposes matching results visually, but keyboard users cannot move focus from the search input into those results. This change provides that missing entry point without replacing the menu's existing keyboard behavior.

Keyboard sequence covered by the test: ArrowDown → ArrowDown → ArrowUp → Enter.

Verification

• SharingDialog tests: 6 suites passed, 40 tests passed, 7 snapshots passed
• package build completed successfully
• changed files pass the repository's ESLint and Prettier checks
• git diff --check is clean

Jira: LIBS-571

Related to #1447

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud