DefaultLabelProvider: Add collapsible SideNavigation accessibility labels

[diyorbek]

Summary

Added type definitions for the collapsible SideNavigation default accesssibility labels.

Links

• Jira
• [TDD](link to Paper doc)
• [Figma](link to Figma file)

Checklist

• [ ] Added unit and Flow Tests
• [ ] Added documentation + accessibility tests
• [ ] Verified accessibility: keyboard & screen reader interaction
• [ ] Checked dark mode, responsiveness, and right-to-left support
• [ ] Checked stakeholder feedback (e.g. Gestalt designers, relevant feature teams)

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