[WC-3363] Fileuploader: dismiss action for validation errors

[yordan-st]

Pull request type

Bug fix (non-breaking change which fixes an issue)

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Description

• Fixed an issue where the "Invalid file format" validation error persisted indefinitely with no way to close it
• Validation errors now auto-clear when a valid file uploads successfully, and each error entry has a dismiss button for manual removal

What should be covered while testing?

• Configure File Uploader with restricted formats (e.g., only .pdf), upload a disallowed file (e.g., .xlsx), confirm "Invalid file format" error appears
• While error is visible, upload an allowed file — verify the error disappears automatically after successful upload
• Upload a disallowed file again, click the dismiss (X) button on the error entry — verify it's removed
• Upload multiple disallowed files, then drop a new batch (mix of valid + invalid) — verify old errors clear and only new rejections show