[Extensions] AlternateColor without item containers

[Avid29]

Fixes #712

PR Type

What kind of change does this PR introduce?

What is the current behavior?

If a ListViewBase does not use a container, for whatever reason, the item's background will not be altered.

What is the new behavior?

If the ItemContainer is not, the item itself will be used if the type is a Control.

PR Checklist

Please check if your PR fulfills the following requirements:

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• Tested code with current supported SDKs(https://github.com/CommunityToolkit/WindowsCommunityToolkit-design-assets)
• Tests for the changes have been added (if applicable)
• Contains NO breaking changes

Other information

[zubinqayam]

Avid29:fix/alt-color6

[Copilot]

Pull request overview

This PR extends the AlternateColor functionality for ListViewBase to support scenarios where no item container is used. When an item container is unavailable, the extension now falls back to using the item itself if it's a Control.

Key changes:

• Added fallback logic to use the item as a Control when ItemContainer is unavailable
• Refactored code across multiple ListViewExtensions files for consistency and modern C# patterns
• Consolidated event cleanup logic with uniquely named unload handlers

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
ListViewExtensions.AlternateRows.cs	Added fallback logic to use args.Item when args.ItemContainer is null, refactored to use modern C# patterns, renamed methods/variables for clarity
ListViewExtensions.Command.cs	Refactored to use expression-bodied members and modern pattern matching for consistency
ListViewExtensions.StretchItemContainer.cs	Implemented previously missing event handlers, changed return type to nullable, refactored for consistency

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[Arlodotexe]

Fully reviewed and tested. Approved!