Add MVVM dependency graph notifications

[bestlux]

Closes #857

Addresses #959
Addresses #906
Addresses #1168

This PR adds explicit dependency graph support for MVVM source generation through a new [DependsOn] attribute for calculated properties.

The new API lets target-side calculated properties declare the source properties they depend on:

[DependsOn(nameof(FirstName), nameof(LastName))]
public string FullName => $"{FirstName} {LastName}";

The source generator then expands those dependencies for generated observable properties, including transitive dependencies declared through [DependsOn]. Existing [NotifyPropertyChangedFor] behavior remains direct-only for compatibility.

This also adds:

• inferred command invalidation for [RelayCommand(CanExecute = nameof(SomeBoolProperty))] when the bool property is generated or graph-derived
• explicit child INotifyPropertyChanged forwarding through [DependsOn(..., NotifyOnSubPropertyChanges = true)]
• diagnostics for invalid dependency sources, self-dependencies, dependency cycles, and invalid child-notification sources
• regression tests for graph expansion, command invalidation, child attach/detach behavior, null handling, initially assigned backing fields, and invalid usage diagnostics

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• Tested code with current supported SDKs
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions

Validation

• dotnet restore .\dotnet.slnx
• dotnet test .\tests\CommunityToolkit.Mvvm.SourceGenerators.Roslyn4120.UnitTests\CommunityToolkit.Mvvm.SourceGenerators.Roslyn4120.UnitTests.csproj --filter "FullyQualifiedName~DependsOn|FullyQualifiedName~ObservableProperty" --no-restore -v minimal
• dotnet test .\tests\CommunityToolkit.Mvvm.Roslyn4120.UnitTests\CommunityToolkit.Mvvm.Roslyn4120.UnitTests.csproj --filter "FullyQualifiedName~DependsOn|FullyQualifiedName~ObservableProperty|FullyQualifiedName~RelayCommand" --no-restore -v minimal
• dotnet build .\dotnet.slnx -c Debug --no-restore
• dotnet test .\dotnet.slnx --no-build -c Debug -l "console;verbosity=minimal"
• git diff --check

Other information

The API intentionally uses a new [DependsOn] attribute rather than changing [NotifyPropertyChangedFor] transitivity, so existing behavior remains compatibility-preserving.

[bestlux]

@bestlux please read the following Contributor License Agreement(CLA). If you agree with the CLA, please reply with the following information.

@dotnet-policy-service agree [company="{your company}"]

Options:

• (default - no company specified) I have sole ownership of intellectual property rights to my Submissions and I am not making Submissions in the course of work for my employer.

@dotnet-policy-service agree

• (when company given) I am making Submissions in the course of work for my employer (or my employer has intellectual property rights in my Submissions by contract or applicable law). I have permission from my employer to make Submissions and enter into this Agreement on behalf of my employer. By signing below, the defined term “You” includes me and my employer.

@dotnet-policy-service agree company="Microsoft"

Contributor License Agreement

@dotnet-policy-service agree