Add Client Script: Mandatory Field Highlighter using proper functions

[Rajat32-op]

PR Description:

• Highlights empty mandatory fields with error messages
• Uses g_form.showFieldMsg() and hideFieldMsg() instead of DOM manipulation
• Includes onLoad script for initial highlighting
• Includes onChange script for real-time updates
• Follows ServiceNow best practices and official APIs

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[ivan-betev]

Thanks for the fix.

The onLoad is fine but onChange won't work as it uses nonexistent fieldName property.

control is a DOM element, not an object with a .fieldName property

Please fix and resubmit or reopen.