fix(auth): aggregate bulk constraint denials into one audit event; ValueTask constraint checks

A bulk command with N denied items wrote N audit events into the bounded,
drop-on-full audit channel, so a low-privilege key could evict every other
pending audit record. Bulk filters now accumulate a ConstraintDenialSummary
(count, constraint names, <=20 sample targets) and write one aggregated
constraint-denied event per command.

The per-item constraint checks are synchronous, so IConstraintEnforcer's
Check* methods now return ValueTask instead of allocating a Task per item.

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Grpc/MxAccessGatewayServiceConstraintTests.cs

@@ -87,8 +87,56 @@ public sealed class MxAccessGatewayServiceConstraintTests
         Assert.True(merged.Results[3].WasSuccessful);
         Assert.Equal("Tank04.Public", merged.Results[3].TagAddress);
         Assert.Equal(104, merged.Results[3].ItemHandle);
-        // Both denied tags recorded.
-        Assert.Equal(2, enforcer.RecordedDenials.Count);
+        // Both denied tags recorded in ONE aggregated bulk denial, not one audit call per item.
+        Assert.Empty(enforcer.RecordedDenials);
+        (string commandKind, ConstraintDenialSummary summary, _) = Assert.Single(enforcer.RecordedBulkDenials);
+        Assert.Equal(nameof(MxCommandKind.AddItemBulk), commandKind);
+        Assert.Equal(2, summary.DeniedCount);
+        Assert.Equal(4, summary.TotalCount);
+        Assert.Equal(["Tank01.Locked", "Tank03.Secret"], summary.SampleTargets);
+    }
+
+    /// <summary>
+    ///     Audit-flood guard: a bulk write whose every entry is denied records exactly one
+    ///     aggregated denial carrying the full denied count, the client correlation id and a sample
+    ///     capped at <see cref="ConstraintDenialSummary.MaxSampleTargets"/> — so one low-privilege
+    ///     key cannot push thousands of events into the bounded, drop-on-full audit channel.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task Invoke_WriteBulk_WithManyDeniedEntries_RecordsOneAggregatedDenial()
+    {
+        const int EntryCount = 500;
+        PredicateConstraintEnforcer enforcer = new() { DenyWriteHandle = (_, _) => true };
+        FakeSessionManager sessionManager = CreateSessionManagerWithSeed();
+        MxAccessGatewayService service = CreateService(sessionManager, enforcer);
+        WriteBulkCommand payload = new() { ServerHandle = 7 };
+        for (int index = 0; index < EntryCount; index++)
+        {
+            payload.Entries.Add(new WriteBulkEntry { ItemHandle = index + 1 });
+        }
+
+        MxCommandRequest request = new()
+        {
+            SessionId = SessionId,
+            ClientCorrelationId = "bulk-flood-1",
+            Command = new MxCommand { Kind = MxCommandKind.WriteBulk, WriteBulk = payload },
+        };
+
+        MxCommandReply reply = await service.Invoke(request, new TestServerCallContext());
+
+        Assert.Equal(0, sessionManager.InvokeCount);
+        Assert.Equal(EntryCount, reply.WriteBulk.Results.Count);
+        Assert.Empty(enforcer.RecordedDenials);
+        (string commandKind, ConstraintDenialSummary summary, string? correlationId) =
+            Assert.Single(enforcer.RecordedBulkDenials);
+        Assert.Equal(nameof(MxCommandKind.WriteBulk), commandKind);
+        Assert.Equal("bulk-flood-1", correlationId);
+        Assert.Equal(EntryCount, summary.DeniedCount);
+        Assert.Equal(EntryCount, summary.TotalCount);
+        Assert.Equal(ConstraintDenialSummary.MaxSampleTargets, summary.SampleTargets.Count);
+        Assert.True(summary.SampleTruncated);
+        Assert.Equal(["write-handle"], summary.ConstraintNames);
     }
 
     /// <summary>