feat: add AOT-friendly WhenActivated for IActivatableView

[glennawatson]

What kind of change does this PR introduce?

Feature - a trim/AOT-safe way to use WhenActivated on an IActivatableView.

What is the new behavior?

ViewForMixins gains AOT-friendly IActivatableView.WhenActivated overloads that are NOT annotated [RequiresUnreferencedCode] and produce no trim/AOT analyzer warnings. Instead of discovering the view's ViewModel by reflection, the caller supplies the ViewModel-change signal directly as an IObservable<object?>:

IDisposable WhenActivated(Func<IEnumerable<IDisposable>> block, IObservable<object?> viewModelChanged)
IDisposable WhenActivated(Action<Action<IDisposable>> block, IObservable<object?> viewModelChanged)
IDisposable WhenActivated(Action<MultipleDisposable> block, IObservable<object?> viewModelChanged)
IDisposable WhenActivated(IObservable<object?> viewModelChanged)

viewModelChanged emits the current ViewModel and every later value (e.g. null on clear). When an emitted value is an IActivatableViewModel, its activator is activated for the duration of the view's activation - exactly like the reflected overloads, but with no reflection.

Usage (AOT-safe): produce viewModelChanged from a reflection-free source such as the source-generated WhenAnyValue in ReactiveUI.SourceGenerators, or a hand-written INotifyPropertyChanged subscription. IObservable<T> is covariant, so a strongly-typed IObservable<TViewModel?> (reference-type ViewModel) can be passed directly.

Why it is AOT-safe:

• The only trim-unsafe dependency in the activation path was the private HandleViewModelActivation calling view.WhenAnyValue<IViewFor, object?>(nameof(view.ViewModel)) (string/expression-based reflection).
• That reflection is now isolated in a tiny [RequiresUnreferencedCode] bridge; the real work moved to a reflection-free HandleViewModelActivation(IObservable<object?>, IObservable<bool>) that both the reflected and the new AOT paths share.
• Fetcher resolution and HandleViewActivation were already reflection-free (shared ResolveActivationEvents).
• Confirmed by building the core ReactiveUI project (which sets IsAotCompatible/EnableTrimAnalyzer/ILLinkTreatWarningsAsErrors) with zero warnings, and by a behavioral test in the PublishAot/TrimMode=full ReactiveUI.AOT.Tests project that calls the new overload with no suppression.

What is the current behavior?

Every IActivatableView.WhenActivated(...) overload is [RequiresUnreferencedCode], because the ViewModel is observed via reflection. AOT/trim publishing flags them and there is no reflection-free alternative (unlike ReactiveUI.Binding).

Closes #4365

What might this PR break?

• Additive public API only; the existing reflected overloads are unchanged in behavior.
• One decision worth a reviewer's eye: the shared fetcher-resolution helper no longer contains the design-mode no-op branch that lived in the core WhenActivated(Func, IViewFor?). That branch was unreachable - the core GetIsDesignMode() extension always returns false at that call site, and the platform packages that provide a design-mode-aware GetIsDesignMode() (WPF) or design-mode-aware fetcher (WinForms) short-circuit before reaching it - so it never executed and runtime behavior is unchanged. Removing it keeps the file free of dead code and reachable at 100% line + branch coverage. If a functional core design-mode fallback is wanted later, it needs a design-mode signal that is actually reachable by the core call site.

Checklist

• I have read the Contribute guide
• Tests have been added or updated (for bug fixes / features)
• Docs have been added or updated (for bug fixes / features)
• Changes target the main branch
• PR title follows Conventional Commits

Additional information

• New public surface recorded in ReactiveUI and ReactiveUI.Reactive PublicAPI.Unshipped.txt for every TFM.
• ViewForMixins.cs is at 100% line and 100% branch coverage, merged across the full ReactiveUI.Tests suite (1973 tests passing); new activation tests also close pre-existing gaps in the file.
• Verified locally on net8.0/net9.0 (core, ReactiveUI.Reactive shim, tests, AOT tests). Windows/platform TFM (WPF/WinForms) validation is left to CI.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
88.7% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.13%. Comparing base (e6088d3) to head (f739f8b).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ReactiveUI.Shared/Activation/ViewForMixins.cs	82.92%	4 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4413      +/-   ##
==========================================
- Coverage   93.13%   93.13%   -0.01%     
==========================================
  Files         345      345              
  Lines       14979    15008      +29     
  Branches     1566     1569       +3     
==========================================
+ Hits        13951    13977      +26     
- Misses        755      757       +2     
- Partials      273      274       +1     

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