Fix/toast store tests

[rabsqueen]

Summary

Added comprehensive test coverage for frontend/stores/toast.store.ts to verify toast creation, dismissal, and bounded stacking behavior under both normal and high-frequency notification scenarios.

Changes

• Added tests for adding new toasts to the store.
• Added coverage for automatic toast dismissal and expiration.
• Added tests for manually dismissing individual toasts.
• Added burst/concurrent toast tests simulating rapid shipment status updates.
• Verified that toast stacking remains bounded and does not allow unbounded accumulation.
• Covered state transitions and cleanup behavior to ensure dismissed toasts are removed correctly.
• Kept the implementation aligned with the existing toast store behavior and frontend testing patterns.

Validation

• Verified individual toast creation and state updates.
• Confirmed auto-dismiss behavior removes expired toasts.
• Confirmed manual dismissal removes the intended toast without affecting active notifications.
• Verified rapid bursts of toast events respect the configured stacking limit.
• Ran the relevant frontend test suite and checks.

Acceptance Criteria

• Test adding toasts.
• Test automatic dismissal.
• Test manual dismissal.
• Test rapid bursts of concurrent toasts.
• Verify toast stacking/dismissal behavior is bounded.
• Confirm dismissed notifications are properly cleaned up.

closes #1370
closes #1372
closes #1373
closes #1374

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[mftee]

No merge conflicts with main. Good fix for the toast store tests - approving.