Extract InfiniteScrollList business logic into InfiniteScrollListViewModel

[jebriede]

Bug

Fixes: https://github.com/NuGet/Client.Engineering/issues/3700

Description

Extract InfiniteScrollList business logic into InfiniteScrollListViewModel

Summary

Extracts all business logic from InfiniteScrollList.xaml.cs (860 lines) into a new InfiniteScrollListViewModel (714 lines), reducing the codebehind to ~200 lines of WPF-specific plumbing. The ViewModel has no WPF type references and is fully unit-testable.

This is the first step in addressing https://github.com/NuGet/Client.Engineering/issues/2539 (business logic in codebehind). Further improvements are tracked in https://github.com/NuGet/Client.Engineering/issues/3702

Design decisions

• ViewModel property naming follows state semantics, not UI semantics: HasUpdatablePackages (not IsUpdateContainerVisible), HasSelectedPackages (not IsUpdateEnabled), SelectionState (not SelectAllChecked), IsUpdateMode (not CheckBoxesEnabled)
• Action delegates for status bar: LoadingStatusBar.Reset()/SetError()/SetCancelled() replace their own DataContext with new objects — fundamentally imperative, can't be XAML bindings. Delegates are wired in the constructor (not Loaded) because PackageManagerControl calls LoadItemsAsync during initialization
• TopLevelPackageCount/TransitivePackageCount stay on the codebehind: They use CollectionViewGroup.ItemCount which reflects filtered counts. LINQ-based counts on the ViewModel would show unfiltered totals — wrong when vulnerability filter is active
• PackageManagerControl accesses _packageList.ViewModel directly: Eliminates passthrough properties on the control. Both types are internal to the same assembly
• Shared converters: Uses existing BooleanToVisibilityConverter and BooleanToHiddenVisibilityConverter from Resources.xaml instead of defining new ones

Testing

New unit tests in InfiniteScrollListViewModelTests.cs covering:

• Constructor validation and default state
• LoadItemsAsync parameter validation and cancellation
• Loading flow: empty results, populated results, clearing on reload
• Selection: select all, deselect, count tracking, tri-state checkbox state
• Filtering: vulnerability filter on/off, loading indicator visibility
• Grouping: HasTransitiveItems detection
• ResolveSelectedItem: by ID, not found, null
• Status bar: ShouldShowStatusBar for cancelled/error/ready/null states
• Collection management: clear, refresh, append

Pre-existing issue found

Group header package counts (TopLevelPackageCount/TransitivePackageCount) intermittently show 0 when the vulnerability filter is active. This is a pre-existing bug on dev — the plain CLR getters have no change notification mechanism, and CollectionViewGroup may not be fully rebuilt when the binding re-evaluates. Tracked separately.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.

[dotnet-policy-service]

This PR contains changes to XAML files. Verify that you've completed the following tasks:

• Complete accessibility testing.
• Complete localizability testing.