fix(grpc): keep the RpcException off the failover warning so no sink can render Status.Detail

Review follow-ups: the warning now carries only the two endpoint URIs and the
status code — attaching the exception object would let any real sink render
its ToString(), which for RpcException includes server-authored Status.Detail.
The leak pin test now also asserts no exception rides on the warning, via a
new WarningExceptions capture on RecordingLogger. Plus two cosmetic nits:
unified ThrowIfNull idiom, documented that FallbackEndpoints mutation after
Validate is unsupported.

Claude-Session: https://claude.ai/code/session_014WNM4vjoVksyyBraTXSZE1

ZB.MOM.WW.Secrets/tests/ZB.MOM.WW.Secrets.Replicator.Grpc.Tests/FailoverSecretsHubReaderTests.cs

@@ -175,6 +175,12 @@ public sealed class FailoverSecretsHubReaderTests
         }
 
         Assert.Equal(1, _logger.WarningCount);
+
+        // The exception object must not ride along either: sinks render an attached exception's
+        // ToString(), which for RpcException includes Status.Detail — the very text the message
+        // template deliberately excludes. A clean message with the exception attached would still
+        // leak in production, so the warning must carry NO exception at all.
+        Assert.All(_logger.WarningExceptions, Assert.Null);
     }
 
     [Fact]

ZB.MOM.WW.Secrets/tests/ZB.MOM.WW.Secrets.Replicator.Grpc.Tests/TestDoubles.cs

@@ -130,6 +130,14 @@ internal sealed class RecordingLogger<TCategory> : ILogger<TCategory>
     /// <summary>Formatted messages logged at <see cref="LogLevel.Warning"/>.</summary>
     public List<string> Warnings { get; } = [];
 
+    /// <summary>
+    /// The exception object attached to each Warning entry, index-aligned with
+    /// <see cref="Warnings"/>. Captured separately from the formatted message because real sinks
+    /// ALSO render an attached exception's <c>ToString()</c> — for <c>RpcException</c> that includes
+    /// <c>Status.Detail</c> — so a leak test that only scans the message would miss that channel.
+    /// </summary>
+    public List<Exception?> WarningExceptions { get; } = [];
+
     /// <summary>Count of information entries.</summary>
     public int InformationCount => Information.Count;
 
@@ -161,6 +169,7 @@ internal sealed class RecordingLogger<TCategory> : ILogger<TCategory>
                 break;
             case LogLevel.Warning:
                 Warnings.Add(message);
+                WarningExceptions.Add(exception);
                 break;
             default:
                 break;