V16 RC: File upload errors should be shown to the user

[iOvergaard]

Description

This pull request refines error handling in the UmbTemporaryFileManager class to improve robustness and user feedback. The key change is the introduction of a more structured check for UmbApiError instances and the addition of a fallback notification mechanism for unhandled server errors.

Error handling improvements:

• src/Umbraco.Web.UI.Client/src/packages/core/temporary-file/temporary-file-manager.class.ts: Replaced a direct instance check of UmbApiError with a static method isUmbApiError for better type safety and readability. Added a fallback to display a user notification for unexpected server errors, ensuring all error cases are handled gracefully.
• Made sure that not only 413 errors but all errors are shown to the user (if UmbApiError is encountered)

[Copilot]

Pull Request Overview

This PR enhances error handling in UmbTemporaryFileManager by replacing the instanceof check with a static type guard and ensuring all server errors surface a user notification.

• Use UmbApiError.isUmbApiError for safer type checking.
• Add a fallback notification for non-413 UmbApiError statuses.

Comments suppressed due to low confidence (1)

src/Umbraco.Web.UI.Client/src/packages/core/temporary-file/temporary-file-manager.class.ts:191

• There are no tests covering the new fallback notification path for non-413 UmbApiError statuses. Consider adding a unit test to verify that #notificationContext.peek is called with the expected error details when an unexpected server error occurs.

if (UmbApiError.isUmbApiError(error)) {