Add AddHealthChecks(Action<HealthCheckServiceOptions>) overload

[KitKeen]

Fixes #8530.

Summary

services.AddHealthChecks() returns an IHealthChecksBuilder, but does not currently expose a convenient way to configure HealthCheckServiceOptions (which DefaultHealthCheckService consumes via IOptions<HealthCheckServiceOptions>). Users today have to call services.Configure<HealthCheckServiceOptions>(...) separately and then services.AddHealthChecks() — two statements instead of one, and the relationship between them is not discoverable from the API surface.

This PR adds a new overload that takes a configuration delegate:

public static IHealthChecksBuilder AddHealthChecks(
    this IServiceCollection services,
    Action<HealthCheckServiceOptions> configureOptions);

Usage:

services.AddHealthChecks(options =>
{
    options.Registrations.Add(new HealthCheckRegistration(
        "marker",
        _ => Task.FromResult(HealthCheckResult.Healthy()),
        failureStatus: null,
        tags: null));
})
.AddCheck<MyHealthCheck>("my-check");

The overload internally calls services.Configure(configureOptions) and then delegates to the existing parameterless AddHealthChecks(), so the idempotency guarantee and the existing service registrations are preserved. The returned IHealthChecksBuilder remains usable for chained registrations.

Changes

• HealthCheckServiceCollectionExtensions.cs — new overload with full XML docs.
• PublicAPI.Unshipped.txt — baselined the new API.
• ServiceCollectionExtensionsTest.cs — added four tests:
  • delegate is invoked on resolved options,
  • returned builder is usable for further registrations alongside the delegate,
  • null services throws ArgumentNullException,
  • null configureOptions throws ArgumentNullException.

[dotnet-policy-service]

Thanks for your PR, @KitKeen. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.