Audit internalized APIs: ensure all global configuration has public non-builder path

[glennawatson]

Context

Raised in #4296 by @ChaseFlorell. The v21+ builder migration internalized several configuration points. DefaultExceptionHandler is the one Chase hit (#4302), but we should audit all global configuration to ensure no other settings were locked behind internal without a public non-builder alternative.

Scope

Audit these global state holders and their configuration paths:

Class/Property	Current Access	Builder Method	Needs Public API?
RxState.DefaultExceptionHandler	internal setter	WithExceptionHandler()	Yes (#4302)
RxSchedulers.MainThreadScheduler	?	WithMainThreadScheduler()	Audit
RxSchedulers.TaskpoolScheduler	?	WithTaskpoolScheduler()	Audit
RxCacheSize	?	WithSmallCacheLimit() / WithBigCacheLimit()	Audit
Suspension host config	?	WithSuspensionHost()	Audit
IMessageBus	?	WithMessageBus()	Audit

Acceptance Criteria

• All global settings that were previously publicly configurable have a public configuration path (builder OR direct)
• Document which settings are builder-only by design vs. which have public alternatives
• No breaking changes for existing builder users

Files to Audit

• src/ReactiveUI/RxState.cs
• src/ReactiveUI/RxSchedulers.cs
• src/ReactiveUI/RxCacheSize.cs
• src/ReactiveUI/Builder/ReactiveUIBuilder.cs
• src/ReactiveUI/Mixins/BuilderMixins.cs

Related: #4296, #4302