Property Editors: Added form control and mandatory support to editors in picker group(Color, Content, Date, Document, Eye dropper, Multi URL).

[engijlr]

Summary

This PR updates multiple property editors to align with the UmbFormControlMixin pattern.

Changes:

Color Picker
-Extended UmbFormControlMixin in both outer and inner elements.
-Added support for mandatory and mandatory message, passed down as required and requiredMessage.
-Bound inner input in the outer.
-Added a validator for missing value in the inner.

Content Picker
-Added mandatory support.
-Prevents validation when readonly is true to avoid unnecessary submission blocking.

Date Picker
-Extended UmbFormControlMixin in the outer.
-Relies on native input validation.
-Bound inner input in the outer.

Document Picker
-Extended UmbFormControlMixin in the outer.
-Added mandatory and mandatory message support.
-Bound inner input in the outer.

Eye Dropper
-Extended UmbFormControlMixin in both outer and inner.
-Added mandatory and mandatory message support.
-Bound inner input in the outer.
-Added validator for missing value.

Multi URL Picker
-Extended UmbFormControlMixin in the inner.
-Added mandatory and mandatory message support.
-Added validator for missing value.

Result

All mentioned property editors now:

• Use UmbFormControlMixin as a single source of truth for form control behavior.
• Support mandatory validation in a consistent way.
• Properly bind their inner input element for form state tracking.

Additional it fixes #17751