feat!: move to StyleSharp analyzers and ReactiveUI 26, add ReactiveUI.Validation.Reactive

[glennawatson]

Summary

This moves ReactiveUI.Validation to ReactiveUI 26 and the StyleSharp analyzer set. It also adds a package for apps that use ReactiveUI.Reactive.

• The build uses StyleSharp, PerformanceSharp and SecuritySharp analyzers in place of StyleCop and Roslynator, with the same .editorconfig as ReactiveUI.Binding.SourceGenerators.
• Every analyzer finding in the library, AndroidX, tests and samples is fixed.
• NuGet packages are updated, including ReactiveUI 26, TUnit 1.72 and DynamicData.
• New package ReactiveUI.Validation.Reactive builds the same source against ReactiveUI.Reactive. Its namespaces start with ReactiveUI.Validation.Reactive, and it takes System.Reactive IScheduler instances.
• New package ReactiveUI.Validation.AndroidX.Reactive does the same for the AndroidX TextInputLayout bindings.
• The Android builds no longer generate a public Resource class. The libraries ship no Android resources.
• When no formatter is passed, the default formatter is found with GetServices. A dependency injection container that throws for an unregistered type falls back to SingleLineFormatter.
• The public API is tracked with PublicApiSharp baselines under PublicAPI/<tfm>/PublicAPI.txt. They replace the Verify approval tests.
• The MinVer floor is 8.0.

Why

• Projects that use ReactiveUI.Reactive could not use ReactiveUI.Validation. Closes [Bug]: Compatibility with ReactiveUI.Reactive 24.0.0 #967.
• With a custom container plugged into the app locator, building a validation binding or a ReactiveValidationObject threw InvalidOperationException for IValidationTextFormatter<string>. Closes [Bug]: Custom DI container with RxUI - Unable to resolve type IValidationTextFormatter #979.
• The repo's analyzers now match the other ReactiveUI repositories.

Breaking changes

• ReactiveUI 26 is required. Its WhenAnyValue needs a reference-type sender.
• BasePropertyValidation<TViewModel, TViewModelProperty> and the ValidationRule extensions need TViewModel to be a class.
• Optional parameters on public methods and constructors are now overloads. Source still compiles; compiled callers must rebuild. Calls that named only a later optional argument, such as strict: false, must pass the earlier ones too.
• ValidationRule throws ArgumentException for an empty message. It still throws ArgumentNullException for a null one.
• The public Resource class is removed from the Android assemblies.
• ReactiveValidationObject and ObservableValidationBase are no longer abstract. Their constructors stay protected.

How this was verified

The full test suite runs twice, once against each package. New tests cover the formatter fallback with a container that throws for unregistered types.

Notes for the reviewer

• Most of the diff is mechanical analyzer output: file headers, one-line summaries, static lambdas and named constants. It is safe to skim.
• Start with ValidationBinding, ValidationTextFormatterResolver and ReactiveUI.Validation.Reactive.csproj.
• Each library file switches its namespace on REACTIVE_SHIM. The library's own namespaces are imported in ReactiveUI.Validation.Usings.props, so the same file compiles into both packages.
• Suppressions, each with a justification: SST1703 on both ViewForExtensions classes, SST2307 on ValidationBinding.ForViewModel, SST2309 on two interface members, CA1005 on ObservableValidation<TViewModel, TValue, TProp>, and SST1436 on the IsExternalInit polyfill. Each fix would break public API or is not possible in C#.
• The ReactiveUI.Binding 9 generator skips WhenAnyValue calls whose selector returns an anonymous type or a private nested type, and the call then throws at run time. Two tests were adjusted for this. It needs a fix in ReactiveUI.Binding.SourceGenerators.

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 100.00%. Comparing base (76e7ee6) to head (807ccdb).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #990   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        23    +2     
  Lines          665       694   +29     
  Branches       114       104   -10     
=========================================
+ Hits           665       694   +29     

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