fix: bind a field control to the view's current view model, so a view can bind before it has one

[glennawatson]

Summary

A generated view binding onto a control the view holds in a field now follows the view's current view model, so a view can bind before it has one.

• Bind, OneWayBind, BindCommand and BindInteraction read the view model through the view's ViewModel property when the view side starts at a field, such as a control named in XAML (v => v.NameBox.Text). They already did when it starts at a property.
• A binding made while the view's ViewModel is null starts once it is set, and follows each replacement, rather than throwing ArgumentNullException.

Why

A binding whose view side started at a field bound to the view model instance it was handed. A view that binds in its constructor or activation, before a router or host assigns its view model, got an ArgumentNullException from the generated binding. A view model assigned later, or replaced on navigation, was never observed. ReactiveUI binds to the view's ViewModel property, so these views worked there. The ReactiveUI.Avalonia example's command page showed it.

Breaking changes

None for a view that sets its view model before binding. A binding onto a field control now observes the view's ViewModel rather than the instance the call names, as a binding onto a property already did.

How this was verified

Runtime tests bind one-way and two-way onto a field control while the view has no view model, then assign and replace it. The field-path command test now sets the view model, as ReactiveUI requires. Ran every Documentation/Pages example, ToPropertyVerification on net8.0 to net11.0, PlatformBindingsVerification -- --verify on net10.0, and a native AOT publish of setup/aot with no warnings.

Notes for the reviewer

TypeDetectionExtractor.ExtractFieldOwner is the change: a field link's owner info now lists the view's ViewModel property, which BindingEmitterHelpers.ResolveViewModelObservation looks for to root the path at the view.

Checklist

• I have read the Contribute guide
• The PR title follows Conventional Commits
• Tests cover this change, or the summary says why they do not
• New or changed public API has XML documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.23%. Comparing base (64d1efa) to head (5b942d4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #158   +/-   ##
=======================================
  Coverage   99.23%   99.23%           
=======================================
  Files         396      396           
  Lines       14458    14467    +9     
  Branches     2063     2065    +2     
=======================================
+ Hits        14348    14357    +9     
  Misses        110      110           

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud