Input Entity Data Picker: wire up client validation

[madsrasmussen]

This PR wires up the client validation so that validation messages are displayed in the UI.

How to test:
Set up amount (min/max) rules in the Data Type config and verify that validation messages are shown accordingly.

[Copilot]

Pull Request Overview

This PR implements client-side validation for the Entity Data Picker property editor by integrating the Umbraco form validation system. The changes enable validation messages to be displayed in the UI when validation constraints are violated.

• Wires up form control validation by adding the input element to the form control system
• Replaces UUI form control mixin with Umbraco's form validation mixin
• Adds type safety and validation for the selection value handling

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
entity-data-picker-property-editor-ui.element.ts	Adds form control registration and validation setup in firstUpdated lifecycle method
input-entity-data.element.ts	Replaces UUI form mixin with Umbraco validation mixin and updates value handling

[AndyButland]

Thanks @madsrasmussen - validation works as expected now. I added a min and max of 1, and see validation errors if saving 0 or 2 items.