Document async options validation sync access breaking change

[Copilot]

.NET 11 RC 1 changes async options validation so synchronous access now fails fast instead of returning an unvalidated options instance. Preview adopters of IAsyncValidateOptions<TOptions> might also need source changes because the interface now derives from IValidateOptions<TOptions>.

• Breaking-change article

  • Added a .NET 11 Extensions compatibility article for the new synchronous access behavior.
  • Covers previous behavior, new behavior, compatibility impact, rationale, recommended migration, and affected APIs.

• Navigation

  • Added the article to the .NET 11 compatibility index.
  • Added the article to the compatibility TOC.

services.AddOptions<MyOptions>()
    .Configure(o => o.Value = 42)
    .ValidateAsync(o => Task.FromResult(o.Value > 0), "Value must be positive.")
    .ValidateOnStart();

await host.StartAsync();

• Fixes [Breaking change]: Synchronous access of async-validated options now throws instead of skipping validation #55794

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Internal previews

File	Preview link
docs/core/compatibility/11.md	Learn preview
docs/core/compatibility/extensions/11/async-options-validation-sync-access.md	Learn preview
docs/core/compatibility/toc.yml	Learn preview

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