Fix BindCommand passing incorrect parameter after assigning new ViewModel to View

[xackus]

What kind of change does this PR introduce?

Fixes #3970.

What is the current behavior?

The command parameter is derived from the original ViewModel passed into BindCommand.

What is the new behavior?

The command parameter is derived from the current ViewModel (IViewFor.ViewModel).

What might this PR break?

Theoretically this could break code that relies on the stale command parameter value.

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

[xackus]

Before this change null was rejected by withParameter.ToObservable(viewModel) anyway.

[xackus]

I found dead code that did the correct thing. I think this was regressed by #2766.

[glennawatson]

Thanks for the PR. Will review over the next day or so. Won't be straight away.

From the quick glance so far looks good though.

[Copilot]

Pull request overview

Fixes a BindCommand inconsistency where the command parameter was being computed from the originally supplied ViewModel instead of the view’s current IViewFor.ViewModel, which shows up when reusing views and reassigning ViewModels.

Changes:

• Update CommandBinderImplementation.BindCommand(..., Expression withParameter, ...) to derive the parameter observable via Reflection.ViewModelWhenAnyValue(...) (i.e., from the current view.ViewModel).
• Remove an internal mixin overload that previously built the parameter observable from the initially passed ViewModel.
• Add a WPF regression test and update WinForms tests to call the binder implementation directly.

Reviewed changes

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

File	Description
src/ReactiveUI/Bindings/Command/CommandBinderImplementation.cs	Computes command parameter from the current view.ViewModel instead of the originally passed ViewModel.
src/ReactiveUI/Bindings/Command/CommandBinderImplementationMixins.cs	Removes the internal overload that created a stale parameter observable from the initially supplied ViewModel.
src/tests/ReactiveUI.Wpf.Tests/Wpf/WpfCommandBindingImplementationTests.cs	Adds a regression test for parameter correctness after ViewModel reassignment.
src/tests/ReactiveUI.WinForms.Tests/winforms/CommandBindingImplementationTests.cs	Updates tests to call BindCommand on the binder implementation instance (no longer using the removed mixin overload).

[glennawatson]

They @xackus if you feel up to it would appreciate it you can evaluate copilot comments. It definitely gets things wrong at times so feel free to write a comment and resolve

Sometimes it has some good suggestions though.

Just a public holiday here so visiting family so limited availability.

[xackus]

Copilot helped uncover another bug by pointing out an issue in my test.
A WPF specific command binding optimization was being applied too aggressively.

[xackus]

There were some copies of the docs that were wrong / out of date.
I didn't really change anything in the documentation, just stitched together a single up to date version, pasted it everywhere, improved formatting, and added a note that the ViewModel parameter is there for type inference.