feat!: announce toasts via shared ARIA live region

[mikeharv]

The basics

• I validated my changes

The details

Resolves

Fixes #9667

Proposed Changes

This PR updates the Toast utility to use the shared announceDynamicAriaState() live region instead of managing its own aria-live region on each toast.

Changes include:

• Removed role="status" and aria-live from the toast DOM element.
• Added a call to announceDynamicAriaState() when a toast is shown.
• Passed the toast message, status role, and configurable assertiveness to the shared live region.
• Removed the Toast.Assertiveness enum and replaced usage with aria.LiveRegionAssertiveness.

Breaking Change

This work removes Blockly.Toast.Assertiveness. Users who have depended upon this will want to switch to the equivalent Blockly.utils.aria.LiveRegionAssertiveness.

Reason for Changes

Previously, each toast managed its own aria-live region. This could lead to duplicate or inconsistent screen reader announcements and duplicated ARIA logic across components.

Using the shared announceDynamicAriaState() utility centralizes live region management, ensures consistent announcement behavior, and aligns toast announcements with other dynamic ARIA announcements in Blockly.

Test Coverage

Tests were updated to reflect the new architecture:

• Verified that toast messages are announced through the shared live region.
• Verified that the live region is assigned the status role.
• Verified that assertiveness defaults to polite and respects the assertiveness option when provided.
• Verified that the toast element itself no longer has aria-live or role="status" to prevent duplicate announcements.

[BenHenning]

Thanks @mikeharv! Changes look good though I do have one concern that will need to be addressed before this can be merged. PTAL.

[BenHenning]

Thanks @mikeharv! No other concerns from me, this looks good!