Add Client Script: Mandatory Field Highlighter with red border

[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
• Follows ServiceNow best practices and official APIs

Pull Request Checklist

Overview

• I have read and understood the CONTRIBUTING.md guidelines
• My pull request has a descriptive title that accurately reflects the changes
• I've included only files relevant to the changes described in the PR title and description
• I've created a new branch in my forked repository for this contribution

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• My code is relevant to ServiceNow developers
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[ivan-betev]

One more finding. The method g_form.getFieldNames is only supported in Service Portal. I'll not close it now, please fix and I'll approve.

[ivan-betev]

Now it makes more sense. Thanks!

I'd suggest to try it in PDI first or cross-check with docs/references to avoid the ping-pong in the future :-)