feat(telemetry): core review fixes (Prometheus+OTLP coexistence, ServiceName validation, null guards) + contract overload note

- Fix #1: Prometheus exporter always wired for metrics; OTLP is additive overlay
  when Exporter == ZbExporter.Otlp so /metrics + MapZbMetrics work in all modes.
- Fix #2: BuildOptions throws ArgumentException when ServiceName is null/whitespace.
- Fix #3: AddZbTelemetry(IHostApplicationBuilder) guard: ThrowIfNull(configure)
  added alongside existing ThrowIfNull(builder).
- Fix #6: Contract doc adds IServiceCollection convenience overload signature.
- Tests: +3 new tests (OtlpExporter still serves /metrics, empty ServiceName throws,
  whitespace ServiceName throws). Total: 7 passed (was 4).

ZB.MOM.WW.Telemetry/src/ZB.MOM.WW.Telemetry/ZbTelemetryExtensions.cs

@@ -25,6 +25,7 @@ public static class ZbTelemetryExtensions
         Action<ZbTelemetryOptions> configure)
     {
         ArgumentNullException.ThrowIfNull(builder);
+        ArgumentNullException.ThrowIfNull(configure);
         builder.Services.AddZbTelemetry(BuildOptions(configure));
         return builder;
     }
@@ -94,20 +95,23 @@ public static class ZbTelemetryExtensions
         ArgumentNullException.ThrowIfNull(configure);
         var options = new ZbTelemetryOptions();
         configure(options);
+        if (string.IsNullOrWhiteSpace(options.ServiceName))
+        {
+            throw new ArgumentException(
+                "ZbTelemetryOptions.ServiceName is required (e.g. \"otopcua\").",
+                nameof(configure));
+        }
         return options;
     }
 
     private static void ApplyMetricsExporter(MeterProviderBuilder metrics, ZbTelemetryOptions options)
     {
-        switch (options.Exporter)
+        // Prometheus is always wired so that /metrics and MapZbMetrics() work regardless of
+        // the exporter setting. OTLP is an additive overlay when explicitly requested.
+        metrics.AddPrometheusExporter();
+        if (options.Exporter == ZbExporter.Otlp)
         {
-            case ZbExporter.Otlp:
-                metrics.AddOtlpExporter(o => ConfigureOtlp(o, options));
-                break;
-            case ZbExporter.Prometheus:
-            default:
-                metrics.AddPrometheusExporter();
-                break;
+            metrics.AddOtlpExporter(o => ConfigureOtlp(o, options));
         }
     }