[Snackbar] Add F6 keyboard shortcut for accessibility

[p-nadeem]

Description

This PR improves the keyboard accessibility of the Snackbar component by adding an F6 keyboard shortcut that moves focus directly to the Snackbar when it is open. This makes it easier for keyboard users to access dynamic snackbars and their interactive elements (such as the close button or links) without having to tab through the entire page.

Changes

• Updated the Snackbar implementation to:
  • Register a keydown listener when the Snackbar is open and handle the F6 key.
  • Focus the Snackbar container when F6 is pressed.
  • Make the Snackbar container focusable by default with tabIndex={0}.
  • Expose the Snackbar container as a live region with role="status" for assistive technologies.
• Updated the Snackbar docs (docs/data/material/components/snackbars/snackbars.md) to document:
  • The new F6 shortcut under the Accessibility → Keyboard navigation section.
  • How the shortcut helps keyboard users reach the Snackbar without traversing the full page.
• Added unit tests for:
  • Focusing the Snackbar when F6 is pressed.
  • Ensuring no focus change occurs when the Snackbar is closed.
  • Verifying role="status" is applied to the Snackbar.
  • Verifying tabIndex="0" is applied so the Snackbar is focusable.
  • Ensuring the behavior works correctly when multiple Snackbars are open.

Related issue

Fixes #45243

Additional

• I have followed (at least) the PR section of the contributing guide.

[mui-bot]

Netlify deploy preview

• docs/data/material/components/snackbars/snackbars.md

Bundle size report

Bundle	Parsed size	Gzip size
@mui/material	🔺+305B(+0.06%)	🔺+59B(+0.04%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

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