fix: use ObservableMixins helpers before RxAppBuilder initialization

[glennawatson]

What kind of change does this PR introduce?

Bug fix.

What is the new behavior?

ObservableMixins no longer forces ReactiveUI initialization. Its pure helpers can be
called before RxAppBuilder...BuildApp() has run:

• WhereNotNull and the internal FromAsyncWithAllNotifications overloads work without
  any builder setup, because none of them touch RxApp, the dependency resolver, or the
  schedulers.
• The static ObservableMixins() constructor that called RxAppBuilder.EnsureInitialized()
  has been removed. Every member of the class was verified to be pure, so no per-member
  initialization guard is required.

The nested observable helpers (WhereNotNullObservable, DeferredValueObservable,
FromAsyncObservable) are now internal so their async-notification, error, and
cancellation paths are exercised by real tests through InternalsVisibleTo. The
unreachable linked-token-source disposal race is isolated behind a single
[ExcludeFromCodeCoverage] helper, leaving no dead code.

What is the current behavior?

Calling a pure helper such as ObservableMixins.WhereNotNull(...) before the app was
built threw TypeInitializationException wrapping
InvalidOperationException: ReactiveUI has not been initialized, because the type
initializer called RxAppBuilder.EnsureInitialized() even though nothing in the class
depended on runtime state.

Closes #4360

What might this PR break?

None. Public API is unchanged (the helpers were already returned as interfaces, and the
nested types moved from private to internal, not public). Behavior only changes for
the previously-failing pre-initialization path, which now succeeds.

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

Coverage: ObservableMixins.cs is at 100% line and 100% branch (77/77 lines), measured
across the full ReactiveUI.Tests suite (1976 tests passing on net9.0).

Sibling-mixin audit (recommended follow-up, not changed here). Every mixin in
src/ReactiveUI.Shared/Mixins carries the same static X() => RxAppBuilder.EnsureInitialized();
pattern. Auditing each:

• MutableDependencyResolverExtensions - pure; operates only on the passed-in
  IMutableDependencyResolver. Forced init appears unnecessary.
• MutableDependencyResolverAOTExtensions - pure; operates only on the passed-in resolver.
  Forced init appears unnecessary.
• ObservedChangedMixins - pure; expression/reflection helpers over IObservedChange, no
  global state. Forced init appears unnecessary.
• ObservableLoggingMixins - pure with respect to the builder flag; uses Splat's .Log()
  (its own locator, which has a default logger). Forced init appears unnecessary.
• ReactiveNotifyPropertyChangedMixins - genuinely needs initialization:
  AppLocator.Current.GetServices<ICreatesObservableForProperty>() requires services that
  BuildApp/WithCoreServices register. Keep an init guard (scoped to the member, per the
  no-class-wide-ctor guidance).
• AutoPersistHelperMixins - genuinely needs initialization: uses
  RxSchedulers.MainThreadScheduler/TaskpoolScheduler and WhenAny, all configured by
  BuildApp. Keep an init guard.

Recommendation: apply the same removal to the four pure mixins in a follow-up, and replace
the class-wide constructor on the two init-dependent mixins with a member-scoped
EnsureInitialized() call rather than a type-initializer side effect.

[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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.19%. Comparing base (e6088d3) to head (eeb2222).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4410      +/-   ##
==========================================
+ Coverage   93.13%   93.19%   +0.05%     
==========================================
  Files         345      345              
  Lines       14979    14978       -1     
  Branches     1566     1565       -1     
==========================================
+ Hits        13951    13959       +8     
+ Misses        755      748       -7     
+ Partials      273      271       -2     

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