Validation: add warning feedback support

[stsrki]

Closes #4804

• Added ValidationStatus.Warning and ValidationWarning.
• Added ValidationFeedback with None, Warning, Success, and Error fragments.
• Warning validation is non-blocking and does not fail Validations.ValidateAll().
• Added warning styles across providers, including Markdown and RichTextEdit extensions.
• Updated demo and validation docs with warning/feedback examples.
• Kept validation feedback rendered in the DOM for accessibility; CSS controls visibility.