Document async options validation in ASP.NET Core 11

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ASP.NET Core 11 adds async options validation during host startup. The options article only covered synchronous validation and ValidateOnStart.

• Async startup validation

  • Added .NET 11+ guidance for async OptionsBuilder<TOptions>.Validate(...) overloads.
  • Clarified that async validators run through ValidateOnStart during IHost.StartAsync.

• Validator patterns

  • Documented IAsyncValidateOptions<TOptions> for dedicated validator classes.
  • Noted registration via IValidateOptions<TOptions> and the synchronous fail-fast path.

• Data annotations and source generation

  • Documented async DataAnnotations participation through DataAnnotationValidateOptions<TOptions>.
  • Added source-generator behavior for [OptionsValidator] emitting ValidateAsync when the validator implements IAsyncValidateOptions<TOptions>.

builder.Services.AddOptions<RemoteServiceOptions>()
    .Bind(builder.Configuration.GetSection(RemoteServiceOptions.SectionName))
    .Validate<IEndpointHealthCheck>(
        async (options, healthCheck, cancellationToken) =>
            await healthCheck.IsAvailableAsync(options.Endpoint, cancellationToken),
        "The configured endpoint isn't available.")
    .ValidateOnStart();

• Scope and limitations
  • Clarified that IOptions<TOptions>.Value, IOptionsSnapshot<TOptions>, and IOptionsMonitor<TOptions> reloads remain synchronous and don't invoke async validators.

• Fixes Document support for async validation for Options #37465

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