test : added accessibility tests for ExportButton component - #11

Summary of What Has Been Done

Created components/ExportButton.accessibility.test.tsx with comprehensive accessibility tests covering:

• Trigger button has aria-haspopup="true" indicating it opens a menu
• Trigger button aria-expanded is false when closed, true when open
• Dropdown has role="menu"
• Each format option has role="menuitem"
• Error message has role="alert" (ARIA live region)
• Trigger button is focusable
• Trigger button is keyboard-activatable via Enter and Space keys
• Export button is disabled while isExporting is true
• PNG, PDF, and SVG menu items are correctly labeled ("Download PNG/PDF/SVG")
• Dropdown closes on Escape key press

Mocked useExportImage hook to control isExporting and error states.

Changes Made

• New file: components/ExportButton.accessibility.test.tsx

Impact it Made

Adds accessibility test coverage for the ExportButton component, ensuring keyboard navigation, ARIA attributes, and screen reader support work correctly.

Closes #8426.

Note: please assign this PR to the tmdeveloper007 account.