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).

components/observability/shared-contract/ZB.MOM.WW.Telemetry.md

@@ -111,6 +111,13 @@ public static class ZbTelemetryExtensions
     public static IServiceCollection AddZbTelemetry(
         this IServiceCollection    services,
         ZbTelemetryOptions         options);
+
+    /// IServiceCollection convenience overload that accepts a configure delegate.
+    /// Equivalent to calling BuildOptions(configure) then AddZbTelemetry(services, options).
+    /// Use when only an IServiceCollection is available but the lambda form is preferred.
+    public static IServiceCollection AddZbTelemetry(
+        this IServiceCollection    services,
+        Action<ZbTelemetryOptions> configure);
 }
 
 /// Builds the shared OTel ResourceBuilder from ZbTelemetryOptions.