DialogService: Add title-less ShowAsync<T> overloads for options and parameters

[danielchalmers]

Description

Adds two title-less generic ShowAsync<T> overloads to IDialogService / DialogService:

• ShowAsync<T>(DialogOptions options)
• ShowAsync<T>(DialogParameters parameters, DialogOptions options)

Today a dialog can be opened with parameters but no title via the existing ShowAsync<T>(DialogParameters), but there is no equivalent for passing DialogOptions (or parameters + options) without a title, so callers have to pass an explicit null/empty title. These overloads round out the generic set and forward using string.Empty, matching the convention already used by ShowAsync<T>() and ShowAsync<T>(DialogParameters).

Non-breaking (default interface methods)

The two new members are declared as default interface methods on IDialogService, so existing hand-written IDialogService implementations keep compiling with no changes; DialogService still provides concrete overrides. This came directly out of dogfooding: adding them as plain abstract members broke a downstream app's hand-rolled IDialogService test double (CS0535). The default bodies avoid that while keeping the overloads on the interface, consistent with the rest of the ShowAsync family. (Happy to switch these to plain abstract members if you'd prefer to treat it as a normal interface-growth breaking change.)

Scope

Limited to the generic ShowAsync<T> family, not the Type-based ShowAsync(Type, ...) family. The generic family already has a title-less overload (ShowAsync<T>(DialogParameters)), so ShowAsync<T>(null) is already ambiguous and these additions introduce no new ambiguity. The Type-based family has no title-less overload today, so adding one would newly make ShowAsync(type, null) ambiguous between string? title and a reference-typed parameter, a source-breaking change. Keeping this to the generic form makes it purely additive.

Tests

Added DialogTests.ShowAsync_TitlelessOverloads_ForwardOptionsAndParameters (both overloads open a dialog with no title and forward options + parameters). Also validated end-to-end by consuming a locally-built package from a downstream .NET MAUI Blazor app (JournalApp): a bUnit test opens a dialog via the native ShowAsync<T>(DialogOptions) (the app's former extension-method workaround removed), and the app's hand-rolled IDialogService implementation compiles unchanged. Verified on net10.0.