refactor(logging): correlation scope + redaction on shared ILogRedactor seam

Move the per-request correlation context and secret redaction off the MEL
mechanism onto the Serilog primitives the shared bootstrap consumes.

- GatewayRequestLoggingMiddlewareExtensions now pushes the correlation
  properties (SessionId / WorkerProcessId / CorrelationId / CommandMethod /
  ClientIdentity) via Serilog LogContext.PushProperty for the request lifetime
  and pops them on completion, replacing MEL ILogger.BeginScope. Header parsing
  and property names are unchanged; GatewayLogScope remains the data holder.
- Add GatewayLogRedactorAdapter : ILogRedactor delegating to the existing
  GatewayLogRedactor policy (mxgw_ bearer tokens / credential-bearing command
  values), registered as a singleton so the shared RedactionEnricher masks
  secrets on every event. Remove the now-dead GatewayLoggerExtensions MEL helper.
- Tests: add GatewayLogRedactorAdapterTests; serialize the four host-building
  test classes into one non-parallel collection (HostBuildingCollection) so the
  process-wide Serilog bootstrap logger is not frozen by two concurrent host
  builds racing in parallel collections.

The net48/x86 worker is untouched.

src/ZB.MOM.WW.MxGateway.Server/GatewayApplication.cs

@@ -84,6 +84,8 @@ public static class GatewayApplication
     /// <c>ZB.MOM.WW.Telemetry.Serilog</c> bootstrap (<see cref="ZbSerilogExtensions.AddZbSerilog"/>).
     /// Sinks and minimum level come from the <c>Serilog</c> configuration section; identity
     /// (<c>SiteId</c>/<c>NodeRole</c>) is read from <c>MxGateway:Telemetry</c> when present.
+    /// Also registers the project's <see cref="ILogRedactor"/> adapter so the shared redaction
+    /// enricher masks gateway secrets on every event.
     /// </summary>
     /// <param name="builder">The web application builder being configured.</param>
     private static void ConfigureSerilog(WebApplicationBuilder builder)
@@ -91,6 +93,8 @@ public static class GatewayApplication
         string? siteId = builder.Configuration["MxGateway:Telemetry:SiteId"];
         string? nodeRole = builder.Configuration["MxGateway:Telemetry:NodeRole"];
 
+        builder.Services.AddSingleton<ILogRedactor, GatewayLogRedactorAdapter>();
+
         builder.AddZbSerilog(options =>
         {
             options.ServiceName = "mxgateway";