AddSiteEventLogHealthMetricsBridge registered via AddHostedService(factory-lambda),
which sets ImplementationFactory and leaves ImplementationType null. The prior
ImplementationType == guard was therefore silently dead — a second call would spin
up a second SiteEventLogFailureCountReporter. Fix: add a private
SiteEventLogHealthMetricsBridgeMarker singleton and guard on its ServiceType instead.
Also corrects the cycle-path comment in both ServiceCollectionExtensions.cs and
SiteEventLogFailureCountReporter.cs: StoreAndForward.csproj does reference
SiteEventLogging.csproj, so the transitive path HealthMonitoring → StoreAndForward →
SiteEventLogging is real, but adding a direct HealthMonitoring → SiteEventLogging
reference would NOT create a cycle (SiteEventLogging has no back-edge to HealthMonitoring).
The Func<long> seam is a coupling-avoidance measure, not a cycle-breaker.
Adds AddSiteEventLogHealthMetricsBridgeTests.AddSiteEventLogHealthMetricsBridge_IsIdempotent_DoesNotDoubleRegister_HostedService
as a regression test (builds provider and asserts exactly one reporter via GetServices<IHostedService>().OfType<T>()).
Joseph Doherty