client/dotnet: backfill XML doc comments to satisfy analyzers

Adds missing <summary>/<param> docs across the .NET client library and its
test suite so CommentChecker reports zero issues. TreatWarningsAsErrors
requires the analyzer surface clean before publishing the NuGet package.

clients/dotnet/ZB.MOM.WW.MxGateway.Client.Tests/GalaxyRepositoryClientTests.cs

@@ -181,6 +181,9 @@ public sealed class GalaxyRepositoryClientTests
         Assert.Contains("repeated page token", exception.Message, StringComparison.Ordinal);
     }
 
+    /// <summary>
+    /// Verifies that DiscoverHierarchyAsync maps typed filter options correctly to the request.
+    /// </summary>
     [Fact]
     public async Task DiscoverHierarchyAsync_WithOptions_MapsTypedFilters()
     {
@@ -212,6 +215,9 @@ public sealed class GalaxyRepositoryClientTests
         Assert.True(request.HistorizedOnly);
     }
 
+    /// <summary>
+    /// Verifies that TestConnectionAsync retries on transient gRPC failures.
+    /// </summary>
     [Fact]
     public async Task TestConnectionAsync_RetriesOnTransientGrpcFailure()
     {