[APDM-1956] Fix DM work on Unity v6.4-beta

[DmitriiFeshchenko]

No description provided.

[Copilot]

Pull request overview

This PR updates two custom UIElements classes to support Unity 6.x's new UIElement registration system while maintaining backward compatibility with older Unity versions. Unity 6.x deprecated the old UxmlFactory pattern in favor of the [UxmlElement] attribute on partial classes.

Changes:

• Added conditional compilation to use [UxmlElement] attribute for Unity 6.x and newer
• Changed classes from regular to partial classes to support the new attribute-based registration
• Wrapped legacy UxmlFactory declarations in conditional compilation for pre-Unity 6.x versions

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Editor/DependencyManager/UIElements/Widgets/Sdk/VersionDropdown/SdkVersionDropdown.cs	Updated UIElement registration to support Unity 6.x using [UxmlElement] attribute and partial class, with conditional compilation for backward compatibility
Editor/DependencyManager/UIElements/Widgets/Sdk/Cards/Shell/SdkCardShell.cs	Updated UIElement registration to support Unity 6.x using [UxmlElement] attribute and partial class, with conditional compilation for backward compatibility

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.